Ruby on Rails Guides: Active Support Core Extensions

# Guides.rubyonrails.org

Skip navigation.

## Active Support Core Extensions

Active Support is the Ruby on Rails component responsible for providing Ruby language extensions, utilities, and other transversal stuff.

It offers a richer bottom-line at the language level, targeted both at the development of Rails applications, and at the development of Ruby on Rails itself.

By referring to this guide you will learn the extensions to the Ruby core classes and modules provided by Active Support.

### Chapters

1. How to Load Core Extensions
   - Stand-Alone Active Support
   - Active Support Within a Ruby on Rails Application
2. Extensions to All Objects
   - `blank?` and `present?`
   - `presence`
   - `duplicable?`
   - `try`
   - `singleton_class`
   - `class_eval(*args, &block)`
   - `acts_like?(duck)`
   - `to_param`
   - `to_query`
   - `with_options`
   - Instance Variables
   - Silencing Warnings, Streams, and Exceptions
   - `in?`
3. Extensions to `Module`
   - `alias_method_chain`
   - Attributes
   - Parents
   - Constants
   - Synchronization
   - Reachable
   - Anonymous
   - Method Delegation
   - Method Names
   - Redefining Methods
4. Extensions to `Class`
   - Class Attributes
   - Class Inheritable Attributes
   - Subclasses & Descendants
5. Extensions to `String`
   - Output Safety
   - `squish`
   - `truncate`
   - `inquiry`
   - Key-based Interpolation
   - `starts_with?` and `ends_with?`
   - `strip_heredoc`
   - Access
   - Inflections
   - Conversions
6. Extensions to `Numeric`
   - Bytes
7. Extensions to `Integer`
   - `multiple_of?`
   - `ordinalize`
8. Extensions to `Float`
   - `round`
9. Extensions to `BigDecimal`
10. Extensions to `Enumerable`
    - `group_by`
    - `sum`
    - `each_with_object`
    - `index_by`
    - `many?`
    - `exclude?`
11. Extensions to `Array`
    - Accessing
    - Random Access
    - Adding Elements
    - Options Extraction
    - Conversions
    - Wrapping
    - Grouping
12. Extensions to `Hash`
    - Conversions
    - Merging
    - Diffing
    - Working with Keys
    - Slicing
    - Extracting
    - Indifferent Access
13. Extensions to `Regexp`
    - `multiline?`
14. Extensions to `Range`
    - `to_s`
    - `step`
    - `include?`
    - `cover?`
    - `overlaps?`
15. Extensions to `Proc`
    - `bind`
16. Extensions to `Date`
    - Calculations
    - Conversions
17. Extensions to `DateTime`
    - Calculations
18. Extensions to `Time`
    - Calculations
    - Time Constructors
19. Extensions to `Process`
    - `daemon`
20. Extensions to `File`
    - `atomic_write`
21. Extensions to `Logger`
    - `around_[level]`
    - `silence`
    - `datetime_format=`
22. Extensions to `NameError`
23. Extensions to `LoadError`

### 1 How to Load Core Extensions

#### 1.1 Stand-Alone Active Support

In order to have a near zero default footprint, Active Support does not load anything by default. It is broken in small pieces so that you may load just what you need, and also has some convenience entry points to load related extensions in one shot, even everything.

Thus, after a simple require like:

    require 'active_support'

objects do not even respond to `blank?`. Let’s see how to load its definition.

##### 1.1.1 Cherry-picking a Definition

The most lightweight way to get `blank?` is to cherry-pick the file that defines it.

For every single method defined as a core extension this guide has a note that says where such a method is defined. In the case of `blank?` the note reads:

Defined in `active_support/core_ext/object/blank.rb`.

That means that this single call is enough:

    require 'active_support/core_ext/object/blank'

Active Support has been carefully revised so that cherry-picking a file loads only strictly needed dependencies, if any.

##### 1.1.2 Loading Grouped Core Extensions

The next level is to simply load all extensions to `Object`. As a rule of thumb, extensions to `SomeClass` are available in one shot by loading `active_support/core_ext/some_class`.

Thus, to load all extensions to `Object` (including `blank?`):

    require 'active_support/core_ext/object'

##### 1.1.3 Loading All Core Extensions

You may prefer just to load all core extensions, there is a file for that:

    require 'active_support/core_ext'

##### 1.1.4 Loading All Active Support

And finally, if you want to have all Active Support available just issue:

    require 'active_support/all'

That does not even put the entire Active Support in memory upfront indeed, some stuff is configured via `autoload`, so it is only loaded if used.

#### 1.2 Active Support Within a Ruby on Rails Application

A Ruby on Rails application loads all Active Support unless `config.active_support.bare` is true. In that case, the application will only load what the framework itself cherry-picks for its own needs, and can still cherry-pick itself at any granularity level, as explained in the previous section.

### 2 Extensions to All Objects

#### 2.1 `blank?` and `present?`

The following values are considered to be blank in a Rails application:

- `nil` and `false`,

- strings composed only of whitespace (see note below),

- empty arrays and hashes, and

- any other object that responds to `empty?` and it is empty.

In Ruby 1.9 the predicate for strings uses the Unicode-aware character class `[:space:]`, so for example U`2029 (paragraph separator) is considered to be whitespace. In Ruby 1.8 whitespace is considered to be `\s` together with the ideographic space U`3000.

Note that numbers are not mentioned, in particular 0 and 0.0 are not blank.

For example, this method from `ActionDispatch::Session::AbstractStore` uses `blank?` for checking whether a session key is present:

    def ensure_session_key!
      if @key.blank?
        raise ArgumentError, 'A key is required...'
      end
    end

The method `present?` is equivalent to `!blank?`. This example is taken from `ActionDispatch::Http::Cache::Response`:

    def set_conditional_cache_control!
      return if self["Cache-Control"].present?
      ...
    end

Defined in `active_support/core_ext/object/blank.rb`.

#### 2.2 `presence`

The `presence` method returns its receiver if `present?`, and `nil` otherwise. It is useful for idioms like this:

    host = config[:host].presence || 'localhost'

Defined in `active_support/core_ext/object/blank.rb`.

#### 2.3 `duplicable?`

A few fundamental objects in Ruby are singletons. For example, in the whole life of a program the integer 1 refers always to the same instance:

    1.object_id                 # => 3
    Math.cos(0).to_i.object_id  # => 3

Hence, there’s no way these objects can be duplicated through `dup` or `clone`:

    true.dup  # => TypeError: can't dup TrueClass

Some numbers which are not singletons are not duplicable either:

    0.0.clone        # => allocator undefined for Float
    (2**1024).clone  # => allocator undefined for Bignum

Active Support provides `duplicable?` to programmatically query an object about this property:

    "".duplicable?     # => true
    false.duplicable?  # => false

By definition all objects are `duplicable?` except `nil`, `false`, `true`, symbols, numbers, and class and module objects.

Any class can disallow duplication removing `dup` and `clone` or raising exceptions from them, only `rescue` can tell whether a given arbitrary object is duplicable. `duplicable?` depends on the hard-coded list above, but it is much faster than `rescue`. Use it only if you know the hard-coded list is enough in your use case.

Defined in `active_support/core_ext/object/duplicable.rb`.

#### 2.4 `try`

Sometimes you want to call a method provided the receiver object is not `nil`, which is something you usually check first. `try` is like `Object#send` except that it returns `nil` if sent to `nil`.

For instance, in this code from `ActiveRecord::ConnectionAdapters::AbstractAdapter` `@logger` could be `nil`, but you save the check and write in an optimistic style:

    def log_info(sql, name, ms)
      if @logger.try(:debug?)
        name = '%s (%.1fms)' % [name || 'SQL', ms]
        @logger.debug(format_log_entry(name, sql.squeeze(' ')))
      end
    end

`try` can also be called without arguments but a block, which will only be executed if the object is not nil:

    @person.try { |p| "#{p.first_name} #{p.last_name}" }

Defined in `active_support/core_ext/object/try.rb`.

#### 2.5 `singleton_class`

The method `singleton_class` returns the singleton class of the receiver:

    String.singleton_class     # => #<Class:String>
    String.new.singleton_class # => #<Class:#<String:0x17a1d1c>>

Fixnums and symbols have no singleton classes, `singleton_class` raises `TypeError` on them. Moreover, the singleton classes of `nil`, `true`, and `false`, are `NilClass`, `TrueClass`, and `FalseClass`, respectively.

Defined in `active_support/core_ext/kernel/singleton_class.rb`.

#### 2.6 `class_eval(*args, &block)`

You can evaluate code in the context of any object’s singleton class using `class_eval`:

    class Proc
      def bind(object)
        block, time = self, Time.now
        object.class_eval do
          method_name = "__bind_#{time.to_i}_#{time.usec}"
          define_method(method_name, &block)
          method = instance_method(method_name)
          remove_method(method_name)
          method
        end.bind(object)
      end
    end

Defined in `active_support/core_ext/kernel/singleton_class.rb`.

#### 2.7 `acts_like?(duck)`

The method `acts_like` provides a way to check whether some class acts like some other class based on a simple convention: a class that provides the same interface as `String` defines

    def acts_like_string?
    end

which is only a marker, its body or return value are irrelevant. Then, client code can query for duck-type-safeness this way:

    some_klass.acts_like?(:string)

Rails has classes that act like `Date` or `Time` and follow this contract.

Defined in `active_support/core_ext/object/acts_like.rb`.

#### 2.8 `to_param`

All objects in Rails respond to the method `to_param`, which is meant to return something that represents them as values in a query string, or as URL fragments.

By default `to_param` just calls `to_s`:

    7.to_param # => "7"

The return value of `to_param` should not be escaped:

    "Tom & Jerry".to_param # => "Tom & Jerry"

Several classes in Rails overwrite this method.

For example `nil`, `true`, and `false` return themselves. `Array#to_param` calls `to_param` on the elements and joins the result with “/”:

    [0, true, String].to_param # => "0/true/String"

Notably, the Rails routing system calls `to_param` on models to get a value for the `:id` placeholder. `ActiveRecord::Base#to_param` returns the `id` of a model, but you can redefine that method in your models. For example, given

    class User
      def to_param
        "#{id}-#{name.parameterize}"
      end
    end

we get:

    user_path(@user) # => "/users/357-john-smith"

Controllers need to be aware of any redefinition of `to_param` because when a request like that comes in “357-john-smith” is the value of `params[:id]`.

Defined in `active_support/core_ext/object/to_param.rb`.

#### 2.9 `to_query`

Except for hashes, given an unescaped `key` this method constructs the part of a query string that would map such key to what `to_param` returns. For example, given

    class User
      def to_param
        "#{id}-#{name.parameterize}"
      end
    end

we get:

    current_user.to_query('user') # => user=357-john-smith

This method escapes whatever is needed, both for the key and the value:

    account.to_query('company[name]')
    # => "company%5Bname%5D=Johnson+%26+Johnson"

so its output is ready to be used in a query string.

Arrays return the result of applying `to_query` to each element with `key[]` as key, and join the result with “&”:

    [3.4, -45.6].to_query('sample')
    # => "sample%5B%5D=3.4&sample%5B%5D=-45.6"

Hashes also respond to `to_query` but with a different signature. If no argument is passed a call generates a sorted series of key/value assignments calling `to_query(key)` on its values. Then it joins the result with “&”:

    {:c => 3, :b => 2, :a => 1}.to_query # => "a=1&b=2&c=3"

The method `Hash#to_query` accepts an optional namespace for the keys:

    {:id => 89, :name => "John Smith"}.to_query('user')
    # => "user%5Bid%5D=89&user%5Bname%5D=John+Smith"

Defined in `active_support/core_ext/object/to_query.rb`.

#### 2.10 `with_options`

The method `with_options` provides a way to factor out common options in a series of method calls.

Given a default options hash, `with_options` yields a proxy object to a block. Within the block, methods called on the proxy are forwarded to the receiver with their options merged. For example, you get rid of the duplication in:

    class Account < ActiveRecord::Base
      has_many :customers, :dependent => :destroy
      has_many :products,  :dependent => :destroy
      has_many :invoices,  :dependent => :destroy
      has_many :expenses,  :dependent => :destroy
    end

this way:

    class Account < ActiveRecord::Base
      with_options :dependent => :destroy do |assoc|
        assoc.has_many :customers
        assoc.has_many :products
        assoc.has_many :invoices
        assoc.has_many :expenses
      end
    end

That idiom may convey grouping to the reader as well. For example, say you want to send a newsletter whose language depends on the user. Somewhere in the mailer you could group locale-dependent bits like this:

    I18n.with_options :locale => user.locale, :scope => "newsletter" do |i18n|
      subject i18n.t :subject
      body    i18n.t :body, :user_name => user.name
    end

Since `with_options` forwards calls to its receiver they can be nested. Each nesting level will merge inherited defaults in addition to their own.

Defined in `active_support/core_ext/object/with_options.rb`.

#### 2.11 Instance Variables

Active Support provides several methods to ease access to instance variables.

##### 2.11.1 `instance_variable_names`

Ruby 1.8 and 1.9 have a method called `instance_variables` that returns the names of the defined instance variables. But they behave differently, in 1.8 it returns strings whereas in 1.9 it returns symbols. Active Support defines `instance_variable_names` as a portable way to obtain them as strings:

    class C
      def initialize(x, y)
        @x, @y = x, y
      end
    end

    C.new(0, 1).instance_variable_names # => ["@y", "@x"]

The order in which the names are returned is unspecified, and it indeed depends on the version of the interpreter.

Defined in `active_support/core_ext/object/instance_variables.rb`.

##### 2.11.2 `instance_values`

The method `instance_values` returns a hash that maps instance variable names without “@” to their corresponding values. Keys are strings both in Ruby 1.8 and 1.9:

    class C
      def initialize(x, y)
        @x, @y = x, y
      end
    end

    C.new(0, 1).instance_values # => {"x" => 0, "y" => 1}

Defined in `active_support/core_ext/object/instance_variables.rb`.

#### 2.12 Silencing Warnings, Streams, and Exceptions

The methods `silence_warnings` and `enable_warnings` change the value of `$VERBOSE` accordingly for the duration of their block, and reset it afterwards:

    silence_warnings { Object.const_set "RAILS_DEFAULT_LOGGER", logger }

You can silence any stream while a block runs with `silence_stream`:

    silence_stream(STDOUT) do
      # STDOUT is silent here
    end

The `quietly` method addresses the common use case where you want to silence STDOUT and STDERR, even in subprocesses:

    quietly { system 'bundle install' }

For example, the railties test suite uses that one in a few places to prevent command messages from being echoed intermixed with the progress status.

Silencing exceptions is also possible with `suppress`. This method receives an arbitrary number of exception classes. If an exception is raised during the execution of the block and is `kind_of?` any of the arguments, `suppress` captures it and returns silently. Otherwise the exception is reraised:

    # If the user is locked the increment is lost, no big deal.
    suppress(ActiveRecord::StaleObjectError) do
      current_user.increment! :visits
    end

Defined in `active_support/core_ext/kernel/reporting.rb`.

#### 2.13 `in?`

The predicate `in?` tests if an object is included in another object. An `ArgumentError` exception will be raised if the argument passed does not respond to `include?`.

Examples of `in?`:

    1.in?([1,2])        # => true
    "lo".in?("hello")   # => true
    25.in?(30..50)      # => false

Defined in `active_support/core_ext/object/inclusion.rb`.

### 3 Extensions to `Module`

#### 3.1 `alias_method_chain`

Using plain Ruby you can wrap methods with other methods, that’s called alias chaining.

For example, let’s say you’d like params to be strings in functional tests, as they are in real requests, but still want the convenience of assigning integers and other kind of values. To accomplish that you could wrap `ActionController::TestCase#process` this way in `test/test_helper.rb`:

    ActionController::TestCase.class_eval do
      # save a reference to the original process method
      alias_method :original_process, :process

      # now redefine process and delegate to original_process
      def process(action, params=nil, session=nil, flash=nil, http_method='GET')
        params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
        original_process(action, params, session, flash, http_method)
      end
    end

That’s the method `get`, `post`, etc., delegate the work to.

That technique has a risk, it could be the case that `:original_process` was taken. To try to avoid collisions people choose some label that characterizes what the chaining is about:

    ActionController::TestCase.class_eval do
      def process_with_stringified_params(...)
        params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
        process_without_stringified_params(action, params, session, flash, http_method)
      end
      alias_method :process_without_stringified_params, :process
      alias_method :process, :process_with_stringified_params
    end

The method `alias_method_chain` provides a shortcut for that pattern:

    ActionController::TestCase.class_eval do
      def process_with_stringified_params(...)
        params = Hash[*params.map {|k, v| [k, v.to_s]}.flatten]
        process_without_stringified_params(action, params, session, flash, http_method)
      end
      alias_method_chain :process, :stringified_params
    end

Rails uses `alias_method_chain` all over the code base. For example validations are added to `ActiveRecord::Base#save` by wrapping the method that way in a separate module specialized in validations.

Defined in `active_support/core_ext/module/aliasing.rb`.

#### 3.2 Attributes

##### 3.2.1 `alias_attribute`

Model attributes have a reader, a writer, and a predicate. You can alias a model attribute having the corresponding three methods defined for you in one shot. As in other aliasing methods, the new name is the first argument, and the old name is the second (my mnemonic is they go in the same order as if you did an assignment):

    class User < ActiveRecord::Base
      # let me refer to the email column as "login",
      # possibly meaningful for authentication code
      alias_attribute :login, :email
    end

Defined in `active_support/core_ext/module/aliasing.rb`.

##### 3.2.2 `attr_accessor_with_default`

The method `attr_accessor_with_default` serves the same purpose as the Ruby macro `attr_accessor` but allows you to set a default value for the attribute:

    class Url
      attr_accessor_with_default :port, 80
    end

    Url.new.port # => 80

The default value can be also specified with a block, which is called in the context of the corresponding object:

    class User
      attr_accessor :name, :surname
      attr_accessor_with_default(:full_name) do
        [name, surname].compact.join(" ")
      end
    end

    u = User.new
    u.name = 'Xavier'
    u.surname = 'Noria'
    u.full_name # => "Xavier Noria"

The result is not cached, the block is invoked in each call to the reader.

You can overwrite the default with the writer:

    url = Url.new
    url.host # => 80
    url.host = 8080
    url.host # => 8080

The default value is returned as long as the attribute is unset. The reader does not rely on the value of the attribute to know whether it has to return the default. It rather monitors the writer: if there’s any assignment the value is no longer considered to be unset.

Active Resource uses this macro to set a default value for the `:primary_key` attribute:

    attr_accessor_with_default :primary_key, 'id'

Defined in `active_support/core_ext/module/attr_accessor_with_default.rb`.

##### 3.2.3 Internal Attributes

When you are defining an attribute in a class that is meant to be subclassed name collisions are a risk. That’s remarkably important for libraries.

Active Support defines the macros `attr_internal_reader`, `attr_internal_writer`, and `attr_internal_accessor`. They behave like their Ruby built-in `attr_*` counterparts, except they name the underlying instance variable in a way that makes collisions less likely.

The macro `attr_internal` is a synonym for `attr_internal_accessor`:

    # library
    class ThirdPartyLibrary::Crawler
      attr_internal :log_level
    end

    # client code
    class MyCrawler < ThirdPartyLibrary::Crawler
      attr_accessor :log_level
    end

In the previous example it could be the case that `:log_level` does not belong to the public interface of the library and it is only used for development. The client code, unaware of the potential conflict, subclasses and defines its own `:log_level`. Thanks to `attr_internal` there’s no collision.

By default the internal instance variable is named with a leading underscore, `@_log_level` in the example above. That’s configurable via `Module.attr_internal_naming_format` though, you can pass any `sprintf`-like format string with a leading `@` and a `%s` somewhere, which is where the name will be placed. The default is `"@_%s"`.

Rails uses internal attributes in a few spots, for examples for views:

    module ActionView
      class Base
        attr_internal :captures
        attr_internal :request, :layout
        attr_internal :controller, :template
      end
    end

Defined in `active_support/core_ext/module/attr_internal.rb`.

##### 3.2.4 Module Attributes

The macros `mattr_reader`, `mattr_writer`, and `mattr_accessor` are analogous to the `cattr_*` macros defined for class. Check Class Attributes.

For example, the dependencies mechanism uses them:

    module ActiveSupport
      module Dependencies
        mattr_accessor :warnings_on_first_load
        mattr_accessor :history
        mattr_accessor :loaded
        mattr_accessor :mechanism
        mattr_accessor :load_paths
        mattr_accessor :load_once_paths
        mattr_accessor :autoloaded_constants
        mattr_accessor :explicitly_unloadable_constants
        mattr_accessor :logger
        mattr_accessor :log_activity
        mattr_accessor :constant_watch_stack
        mattr_accessor :constant_watch_stack_mutex
      end
    end

Defined in `active_support/core_ext/module/attribute_accessors.rb`.

#### 3.3 Parents

##### 3.3.1 `parent`

The `parent` method on a nested named module returns the module that contains its corresponding constant:

    module X
      module Y
        module Z
        end
      end
    end
    M = X::Y::Z

    X::Y::Z.parent # => X::Y
    M.parent       # => X::Y

If the module is anonymous or belongs to the top-level, `parent` returns `Object`.

Note that in that case `parent_name` returns `nil`.

Defined in `active_support/core_ext/module/introspection.rb`.

##### 3.3.2 `parent_name`

The `parent_name` method on a nested named module returns the fully-qualified name of the module that contains its corresponding constant:

    module X
      module Y
        module Z
        end
      end
    end
    M = X::Y::Z

    X::Y::Z.parent_name # => "X::Y"
    M.parent_name       # => "X::Y"

For top-level or anonymous modules `parent_name` returns `nil`.

Note that in that case `parent` returns `Object`.

Defined in `active_support/core_ext/module/introspection.rb`.

##### 3.3.3 `parents`

The method `parents` calls `parent` on the receiver and upwards until `Object` is reached. The chain is returned in an array, from bottom to top:

    module X
      module Y
        module Z
        end
      end
    end
    M = X::Y::Z

    X::Y::Z.parents # => [X::Y, X, Object]
    M.parents       # => [X::Y, X, Object]

Defined in `active_support/core_ext/module/introspection.rb`.

#### 3.4 Constants

The method `local_constants` returns the names of the constants that have been defined in the receiver module:

    module X
      X1 = 1
      X2 = 2
      module Y
        Y1 = :y1
        X1 = :overrides_X1_above
      end
    end

    X.local_constants    # => ["X2", "X1", "Y"], assumes Ruby 1.8
    X::Y.local_constants # => ["X1", "Y1"], assumes Ruby 1.8

The names are returned as strings in Ruby 1.8, and as symbols in Ruby 1.9. The method `local_constant_names` always returns strings.

This method returns precise results in Ruby 1.9. In older versions of Ruby, however, it may miss some constants in case the same constant exists in the receiver module as well as in any of its ancestors and both constants point to the same object (objects are compared using `Object#object_id`).

Defined in `active_support/core_ext/module/introspection.rb`.

##### 3.4.1 Qualified Constant Names

The standard methods `const_defined?`, `const_get` , and `const_set` accept bare constant names. Active Support extends this API to be able to pass relative qualified constant names.

The new methods are `qualified_const_defined?`, `qualified_const_get`, and `qualified_const_set`. Their arguments are assumed to be qualified constant names relative to their receiver:

    Object.qualified_const_defined?("Math::PI")       # => true
    Object.qualified_const_get("Math::PI")            # => 3.141592653589793
    Object.qualified_const_set("Math::Phi", 1.618034) # => 1.618034

Arguments may be bare constant names:

    Math.qualified_const_get("E") # => 2.718281828459045

These methods are analogous to their builtin counterparts. In particular, `qualified_constant_defined?` accepts an optional second argument in 1.9 to be able to say whether you want the predicate to look in the ancestors. This flag is taken into account for each constant in the expression while walking down the path.

For example, given

    module M
      X = 1
    end

    module N
      class C
        include M
      end
    end

`qualified_const_defined?` behaves this way:

    N.qualified_const_defined?("C::X", false) # => false (1.9 only)
    N.qualified_const_defined?("C::X", true)  # => true (1.9 only)
    N.qualified_const_defined?("C::X")        # => false in 1.8, true in 1.9

As the last example implies, in 1.9 the second argument defaults to true, as in `const_defined?`.

For coherence with the builtin methods only relative paths are accepted. Absolute qualified constant names like `::Math::PI` raise `NameError`.

Defined in `active_support/core_ext/module/qualified_const.rb`.

#### 3.5 Synchronization

The `synchronize` macro declares a method to be synchronized:

    class Counter
      @@mutex = Mutex.new
      attr_reader :value

      def initialize
        @value = 0
      end

      def incr
        @value += 1 # non-atomic
      end
      synchronize :incr, :with => '@@mutex'
    end

The method receives the name of an action, and a `:with` option with code. The code is evaluated in the context of the receiver each time the method is invoked, and it should evaluate to a `Mutex` instance or any other object that responds to `synchronize` and accepts a block.

Defined in `active_support/core_ext/module/synchronization.rb`.

#### 3.6 Reachable

A named module is reachable if it is stored in its corresponding constant. It means you can reach the module object via the constant.

That is what ordinarily happens, if a module is called “M”, the `M` constant exists and holds it:

    module M
    end

    M.reachable? # => true

But since constants and modules are indeed kind of decoupled, module objects can become unreachable:

    module M
    end

    orphan = Object.send(:remove_const, :M)

    # The module object is orphan now but it still has a name.
    orphan.name # => "M"

    # You cannot reach it via the constant M because it does not even exist.
    orphan.reachable? # => false

    # Let's define a module called "M" again.
    module M
    end

    # The constant M exists now again, and it stores a module
    # object called "M", but it is a new instance.
    orphan.reachable? # => false

Defined in `active_support/core_ext/module/reachable.rb`.

#### 3.7 Anonymous

A module may or may not have a name:

    module M
    end
    M.name # => "M"

    N = Module.new
    N.name # => "N"

    Module.new.name # => "" in 1.8, nil in 1.9

You can check whether a module has a name with the predicate `anonymous?`:

    module M
    end
    M.anonymous? # => false

    Module.new.anonymous? # => true

Note that being unreachable does not imply being anonymous:

    module M
    end

    m = Object.send(:remove_const, :M)

    m.reachable? # => false
    m.anonymous? # => false

though an anonymous module is unreachable by definition.

Defined in `active_support/core_ext/module/anonymous.rb`.

#### 3.8 Method Delegation

The macro `delegate` offers an easy way to forward methods.

Let’s imagine that users in some application have login information in the `User` model but name and other data in a separate `Profile` model:

    class User < ActiveRecord::Base
      has_one :profile
    end

With that configuration you get a user’s name via his profile, `user.profile.name`, but it could be handy to still be able to access such attribute directly:

    class User < ActiveRecord::Base
      has_one :profile

      def name
        profile.name
      end
    end

That is what `delegate` does for you:

    class User < ActiveRecord::Base
      has_one :profile

      delegate :name, :to => :profile
    end

It is shorter, and the intention more obvious.

The method must be public in the target.

The `delegate` macro accepts several methods:

    delegate :name, :age, :address, :twitter, :to => :profile

When interpolated into a string, the `:to` option should become an expression that evaluates to the object the method is delegated to. Typically a string or symbol. Such an expression is evaluated in the context of the receiver:

    # delegates to the Rails constant
    delegate :logger, :to => :Rails

    # delegates to the receiver's class
    delegate :table_name, :to => 'self.class'

If the `:prefix` option is `true` this is less generic, see below.

By default, if the delegation raises `NoMethodError` and the target is `nil` the exception is propagated. You can ask that `nil` is returned instead with the `:allow_nil` option:

    delegate :name, :to => :profile, :allow_nil => true

With `:allow_nil` the call `user.name` returns `nil` if the user has no profile.

The option `:prefix` adds a prefix to the name of the generated method. This may be handy for example to get a better name:

    delegate :street, :to => :address, :prefix => true

The previous example generates `address_street` rather than `street`.

Since in this case the name of the generated method is composed of the target object and target method names, the `:to` option must be a method name.

A custom prefix may also be configured:

    delegate :size, :to => :attachment, :prefix => :avatar

In the previous example the macro generates `avatar_size` rather than `size`.

Defined in `active_support/core_ext/module/delegation.rb`

#### 3.9 Method Names

The builtin methods `instance_methods` and `methods` return method names as strings or symbols depending on the Ruby version. Active Support defines `instance_method_names` and `method_names` to be equivalent to them, respectively, but always getting strings back.

For example, `ActionView::Helpers::FormBuilder` knows this array difference is going to work no matter the Ruby version:

    self.field_helpers = (FormHelper.instance_method_names - ['form_for'])

Defined in `active_support/core_ext/module/method_names.rb`

#### 3.10 Redefining Methods

There are cases where you need to define a method with `define_method`, but don’t know whether a method with that name already exists. If it does, a warning is issued if they are enabled. No big deal, but not clean either.

The method `redefine_method` prevents such a potential warning, removing the existing method before if needed. Rails uses it in a few places, for instance when it generates an association’s API:

    redefine_method("#{reflection.name}=") do |new_value|
      association = association_instance_get(reflection.name)

      if association.nil? || association.target != new_value
        association = association_proxy_class.new(self, reflection)
      end

      association.replace(new_value)
      association_instance_set(reflection.name, new_value.nil? ? nil : association)
    end

Defined in `active_support/core_ext/module/remove_method.rb`

### 4 Extensions to `Class`

#### 4.1 Class Attributes

##### 4.1.1 `class_attribute`

The method `class_attribute` declares one or more inheritable class attributes that can be overridden at any level down the hierarchy.

    class A
      class_attribute :x
    end

    class B < A; end

    class C < B; end

    A.x = :a
    B.x # => :a
    C.x # => :a

    B.x = :b
    A.x # => :a
    C.x # => :b

    C.x = :c
    A.x # => :a
    B.x # => :b

For example `ActionMailer::Base` defines:

    class_attribute :default_params
    self.default_params = {
      :mime_version => "1.0",
      :charset      => "UTF-8",
      :content_type => "text/plain",
      :parts_order  => [ "text/plain", "text/enriched", "text/html" ]
    }.freeze

They can be also accessed and overridden at the instance level.

    A.x = 1

    a1 = A.new
    a2 = A.new
    a2.x = 2

    a1.x # => 1, comes from A
    a2.x # => 2, overridden in a2

The generation of the writer instance method can be prevented by setting the option `:instance_writer` to `false`.

    module ActiveRecord
      class Base
        class_attribute :table_name_prefix, :instance_writer => false
        self.table_name_prefix = ""
      end
    end

A model may find that option useful as a way to prevent mass-assignment from setting the attribute.

The generation of the reader instance method can be prevented by setting the option `:instance_reader` to `false`.

    class A
      class_attribute :x, :instance_reader => false
    end

    A.new.x = 1 # NoMethodError

For convenience `class_attribute` also defines an instance predicate which is the double negation of what the instance reader returns. In the examples above it would be called `x?`.

When `:instance_reader` is `false`, the instance predicate returns a `NoMethodError` just like the reader method.

Defined in `active_support/core_ext/class/attribute.rb`

##### 4.1.2 `cattr_reader`, `cattr_writer`, and `cattr_accessor`

The macros `cattr_reader`, `cattr_writer`, and `cattr_accessor` are analogous to their `attr_*` counterparts but for classes. They initialize a class variable to `nil` unless it already exists, and generate the corresponding class methods to access it:

    class MysqlAdapter < AbstractAdapter
      # Generates class methods to access @@emulate_booleans.
      cattr_accessor :emulate_booleans
      self.emulate_booleans = true
    end

Instance methods are created as well for convenience, they are just proxies to the class attribute. So, instances can change the class attribute, but cannot override it as it happens with `class_attribute` (see above). For example given

    module ActionView
      class Base
        cattr_accessor :field_error_proc
        @@field_error_proc = Proc.new{ ... }
      end
    end

we can access `field_error_proc` in views.

The generation of the reader instance method can be prevented by setting `:instance_reader` to `false` and the generation of the writer instance method can be prevented by setting `:instance_writer` to `false`. Generation of both methods can be prevented by setting `:instance_accessor` to `false`. In all cases, the value must be exactly `false` and not any false value.

    module A
      class B
        # No first_name instance reader is generated.
        cattr_accessor :first_name, :instance_reader => false
        # No last_name= instance writer is generated.
        cattr_accessor :last_name, :instance_writer => false
        # No surname instance reader or surname= writer is generated.
        cattr_accessor :surname, :instance_accessor => false
      end
    end

A model may find it useful to set `:instance_accessor` to `false` as a way to prevent mass-assignment from setting the attribute.

Defined in `active_support/core_ext/class/attribute_accessors.rb`.

#### 4.2 Class Inheritable Attributes

Class Inheritable Attributes are deprecated. It’s recommended that you use `Class#class_attribute` instead.

Class variables are shared down the inheritance tree. Class instance variables are not shared, but they are not inherited either. The macros `class_inheritable_reader`, `class_inheritable_writer`, and `class_inheritable_accessor` provide accessors for class-level data which is inherited but not shared with children:

    module ActionController
      class Base
        # FIXME: REVISE/SIMPLIFY THIS COMMENT.
        # The value of allow_forgery_protection is inherited,
        # but its value in a particular class does not affect
        # the value in the rest of the controllers hierarchy.
        class_inheritable_accessor :allow_forgery_protection
      end
    end

They accomplish this with class instance variables and cloning on subclassing, there are no class variables involved. Cloning is performed with `dup` as long as the value is duplicable.

There are some variants specialised in arrays and hashes:

    class_inheritable_array
    class_inheritable_hash

Those writers take any inherited array or hash into account and extend them rather than overwrite them.

As with vanilla class attribute accessors these macros create convenience instance methods for reading and writing. The generation of the writer instance method can be prevented setting `:instance_writer` to `false` (not any false value, but exactly `false`):

    module ActiveRecord
      class Base
        class_inheritable_accessor :default_scoping, :instance_writer => false
      end
    end

Since values are copied when a subclass is defined, if the base class changes the attribute after that, the subclass does not see the new value. That’s the point.

Defined in `active_support/core_ext/class/inheritable_attributes.rb`.

#### 4.3 Subclasses & Descendants

##### 4.3.1 `subclasses`

The `subclasses` method returns the subclasses of the receiver:

    class C; end
    C.subclasses # => []

    class B < C; end
    C.subclasses # => [B]

    class A < B; end
    C.subclasses # => [B]

    class D < C; end
    C.subclasses # => [B, D]

The order in which these classes are returned is unspecified.

This method is redefined in some Rails core classes but should be all compatible in Rails 3.1.

Defined in `active_support/core_ext/class/subclasses.rb`.

##### 4.3.2 `descendants`

The `descendants` method returns all classes that are `<` than its receiver:

    class C; end
    C.descendants # => []

    class B < C; end
    C.descendants # => [B]

    class A < B; end
    C.descendants # => [B, A]

    class D < C; end
    C.descendants # => [B, A, D]

The order in which these classes are returned is unspecified.

Defined in `active_support/core_ext/class/subclasses.rb`.

### 5 Extensions to `String`

#### 5.1 Output Safety

##### 5.1.1 Motivation

Inserting data into HTML templates needs extra care. For example you can’t just interpolate `@review.title` verbatim into an HTML page. On one hand if the review title is “Flanagan & Matz rules!” the output won’t be well-formed because an ampersand has to be escaped as “&amp;”. On the other hand, depending on the application that may be a big security hole because users can inject malicious HTML setting a hand-crafted review title. Check out the section about cross-site scripting in the Security guide for further information about the risks.

##### 5.1.2 Safe Strings

Active Support has the concept of (html) safe strings since Rails 3. A safe string is one that is marked as being insertable into HTML as is. It is trusted, no matter whether it has been escaped or not.

Strings are considered to be unsafe by default:

    "".html_safe? # => false

You can obtain a safe string from a given one with the `html_safe` method:

    s = "".html_safe
    s.html_safe? # => true

It is important to understand that `html_safe` performs no escaping whatsoever, it is just an assertion:

    s = "<script>...</script>".html_safe
    s.html_safe? # => true
    s            # => "<script>...</script>"

It is your responsibility to ensure calling `html_safe` on a particular string is fine.

If you append onto a safe string, either in-place with `concat`/`<<`, or with `+`, the result is a safe string. Unsafe arguments are escaped:

    "".html_safe + "<" # => "&lt;"

Safe arguments are directly appended:

    "".html_safe + "<".html_safe # => "<"

These methods should not be used in ordinary views. In Rails 3 unsafe values are automatically escaped:

    <%= @review.title %> <%# fine in Rails 3, escaped if needed %>

To insert something verbatim use the `raw` helper rather than calling `html_safe`:

    <%= raw @cms.current_template %> <%# inserts @cms.current_template as is %>

or, equivalently, use `<%==`:

    <%== @cms.current_template %> <%# inserts @cms.current_template as is %>

The `raw` helper calls `html_safe` for you:

    def raw(stringish)
      stringish.to_s.html_safe
    end

Defined in `active_support/core_ext/string/output_safety.rb`.

##### 5.1.3 Transformation

As a rule of thumb, except perhaps for concatenation as explained above, any method that may change a string gives you an unsafe string. These are `donwcase`, `gsub`, `strip`, `chomp`, `underscore`, etc.

In the case of in-place transformations like `gsub!` the receiver itself becomes unsafe.

The safety bit is lost always, no matter whether the transformation actually changed something.

##### 5.1.4 Conversion and Coercion

Calling `to_s` on a safe string returns a safe string, but coercion with `to_str` returns an unsafe string.

##### 5.1.5 Copying

Calling `dup` or `clone` on safe strings yields safe strings.

#### 5.2 `squish`

The method `squish` strips leading and trailing whitespace, and substitutes runs of whitespace with a single space each:

    " \n  foo\n\r \t bar \n".squish # => "foo bar"

There’s also the destructive version `String#squish!`.

Defined in `active_support/core_ext/string/filters.rb`.

#### 5.3 `truncate`

The method `truncate` returns a copy of its receiver truncated after a given `length`:

    "Oh dear! Oh dear! I shall be late!".truncate(20)
    # => "Oh dear! Oh dear!..."

Ellipsis can be customized with the `:omission` option:

    "Oh dear! Oh dear! I shall be late!".truncate(20, :omission => '&hellip;')
    # => "Oh dear! Oh &hellip;"

Note in particular that truncation takes into account the length of the omission string.

Pass a `:separator` to truncate the string at a natural break:

    "Oh dear! Oh dear! I shall be late!".truncate(18)
    # => "Oh dear! Oh dea..."
    "Oh dear! Oh dear! I shall be late!".truncate(18, :separator => ' ')
    # => "Oh dear! Oh..."

In the above example “dear” gets cut first, but then `:separator` prevents it.

The option `:separator` can’t be a regexp.

Defined in `active_support/core_ext/string/filters.rb`.

#### 5.4 `inquiry`

The `inquiry` method converts a string into a `StringInquirer` object making equality checks prettier.

    "production".inquiry.production? # => true
    "active".inquiry.inactive?       # => false

#### 5.5 Key-based Interpolation

In Ruby 1.9 the `%` string operator supports key-based interpolation, both formatted and unformatted:

    "Total is %<total>.02f" % {:total => 43.1}  # => Total is 43.10
    "I say %{foo}" % {:foo => "wadus"}          # => "I say wadus"
    "I say %{woo}" % {:foo => "wadus"}          # => KeyError

Active Support adds that functionality to `%` in previous versions of Ruby.

Defined in `active_support/core_ext/string/interpolation.rb`.

#### 5.6 `starts_with?` and `ends_with?`

Active Support defines 3rd person aliases of `String#start_with?` and `String#end_with?`:

    "foo".starts_with?("f") # => true
    "foo".ends_with?("o")   # => true

Defined in `active_support/core_ext/string/starts_ends_with.rb`.

#### 5.7 `strip_heredoc`

The method `strip_heredoc` strips indentation in heredocs.

For example in

    if options[:usage]
      puts <<-USAGE.strip_heredoc
        This command does such and such.

        Supported options are:
          -h         This message
          ...
      USAGE
    end

the user would see the usage message aligned against the left margin.

Technically, it looks for the least indented line in the whole string, and removes that amount of leading whitespace.

Defined in `active_support/core_ext/string/strip.rb`.

#### 5.8 Access

##### 5.8.1 `at(position)`

Returns the character of the string at position `position`:

    "hello".at(0)  # => "h"
    "hello".at(4)  # => "o"
    "hello".at(-1) # => "o"
    "hello".at(10) # => ERROR if < 1.9, nil in 1.9

Defined in `active_support/core_ext/string/access.rb`.

##### 5.8.2 `from(position)`

Returns the substring of the string starting at position `position`:

    "hello".from(0)  # => "hello"
    "hello".from(2)  # => "llo"
    "hello".from(-2) # => "lo"
    "hello".from(10) # => "" if < 1.9, nil in 1.9

Defined in `active_support/core_ext/string/access.rb`.

##### 5.8.3 `to(position)`

Returns the substring of the string up to position `position`:

    "hello".to(0)  # => "h"
    "hello".to(2)  # => "hel"
    "hello".to(-2) # => "hell"
    "hello".to(10) # => "hello"

Defined in `active_support/core_ext/string/access.rb`.

##### 5.8.4 `first(limit = 1)`

The call `str.first(n)` is equivalent to `str.to(n-1)` if `n` > 0, and returns an empty string for `n` == 0.

Defined in `active_support/core_ext/string/access.rb`.

##### 5.8.5 `last(limit = 1)`

The call `str.last(n)` is equivalent to `str.from(-n)` if `n` > 0, and returns an empty string for `n` == 0.

Defined in `active_support/core_ext/string/access.rb`.

#### 5.9 Inflections

##### 5.9.1 `pluralize`

The method `pluralize` returns the plural of its receiver:

    "table".pluralize     # => "tables"
    "ruby".pluralize      # => "rubies"
    "equipment".pluralize # => "equipment"

As the previous example shows, Active Support knows some irregular plurals and uncountable nouns. Built-in rules can be extended in `config/initializers/inflections.rb`. That file is generated by the `rails` command and has instructions in comments.

`pluralize` can also take an optional `count` parameter. If `count == 1` the singular form will be returned. For any other value of `count` the plural form will be returned:

    "dude".pluralize(0) # => "dudes"
    "dude".pluralize(1) # => "dude"
    "dude".pluralize(2) # => "dudes"

Active Record uses this method to compute the default table name that corresponds to a model:

    # active_record/base.rb
    def undecorated_table_name(class_name = base_class.name)
      table_name = class_name.to_s.demodulize.underscore
      table_name = table_name.pluralize if pluralize_table_names
      table_name
    end

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.2 `singularize`

The inverse of `pluralize`:

    "tables".singularize    # => "table"
    "rubies".singularize    # => "ruby"
    "equipment".singularize # => "equipment"

Associations compute the name of the corresponding default associated class using this method:

    # active_record/reflection.rb
    def derive_class_name
      class_name = name.to_s.camelize
      class_name = class_name.singularize if collection?
      class_name
    end

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.3 `camelize`

The method `camelize` returns its receiver in camel case:

    "product".camelize    # => "Product"
    "admin_user".camelize # => "AdminUser"

As a rule of thumb you can think of this method as the one that transforms paths into Ruby class or module names, where slashes separate namespaces:

    "backoffice/session".camelize # => "Backoffice::Session"

For example, Action Pack uses this method to load the class that provides a certain session store:

    # action_controller/metal/session_management.rb
    def session_store=(store)
      if store == :active_record_store
        self.session_store = ActiveRecord::SessionStore
      else
        @@session_store = store.is_a?(Symbol) ?
          ActionDispatch::Session.const_get(store.to_s.camelize) :
          store
      end
    end

`camelize` accepts an optional argument, it can be `:upper` (default), or `:lower`. With the latter the first letter becomes lowercase:

    "visual_effect".camelize(:lower) # => "visualEffect"

That may be handy to compute method names in a language that follows that convention, for example JavaScript.

As a rule of thumb you can think of `camelize` as the inverse of `underscore`, though there are cases where that does not hold: `“SSLError”.underscore.camelize` gives back `“SslError”`. To support cases such as this, Active Support allows you to specify acronyms in `config/initializers/inflections.rb`:

    ActiveSupport::Inflector.inflections do |inflect|
      inflect.acronym 'SSL'
    end

    "SSLError".underscore.camelize #=> "SSLError"

`camelize` is aliased to `camelcase`.

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.4 `underscore`

The method `underscore` goes the other way around, from camel case to paths:

    "Product".underscore   # => "product"
    "AdminUser".underscore # => "admin_user"

Also converts “::” back to “/”:

    "Backoffice::Session".underscore # => "backoffice/session"

and understands strings that start with lowercase:

    "visualEffect".underscore # => "visual_effect"

`underscore` accepts no argument though.

Rails class and module autoloading uses `underscore` to infer the relative path without extension of a file that would define a given missing constant:

    # active_support/dependencies.rb
    def load_missing_constant(from_mod, const_name)
      ...
      qualified_name = qualified_name_for from_mod, const_name
      path_suffix = qualified_name.underscore
      ...
    end

As a rule of thumb you can think of `underscore` as the inverse of `camelize`, though there are cases where that does not hold. For example, `“SSLError”.underscore.camelize` gives back `“SslError”`.

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.5 `titleize`

The method `titleize` capitalizes the words in the receiver:

    "alice in wonderland".titleize # => "Alice In Wonderland"
    "fermat's enigma".titleize     # => "Fermat's Enigma"

`titleize` is aliased to `titlecase`.

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.6 `dasherize`

The method `dasherize` replaces the underscores in the receiver with dashes:

    "name".dasherize         # => "name"
    "contact_data".dasherize # => "contact-data"

The XML serializer of models uses this method to dasherize node names:

    # active_model/serializers/xml.rb
    def reformat_name(name)
      name = name.camelize if camelize?
      dasherize? ? name.dasherize : name
    end

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.7 `demodulize`

Given a string with a qualified constant name, `demodulize` returns the very constant name, that is, the rightmost part of it:

    "Product".demodulize                        # => "Product"
    "Backoffice::UsersController".demodulize    # => "UsersController"
    "Admin::Hotel::ReservationUtils".demodulize # => "ReservationUtils"

Active Record for example uses this method to compute the name of a counter cache column:

    # active_record/reflection.rb
    def counter_cache_column
      if options[:counter_cache] == true
        "#{active_record.name.demodulize.underscore.pluralize}_count"
      elsif options[:counter_cache]
        options[:counter_cache]
      end
    end

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.8 `deconstantize`

Given a string with a qualified constant reference expression, `deconstantize` removes the rightmost segment, generally leaving the name of the constant’s container:

    "Product".deconstantize                        # => ""
    "Backoffice::UsersController".deconstantize    # => "Backoffice"
    "Admin::Hotel::ReservationUtils".deconstantize # => "Admin::Hotel"

Active Support for example uses this method in `Module#qualified_const_set`:

    def qualified_const_set(path, value)
      QualifiedConstUtils.raise_if_absolute(path)

      const_name = path.demodulize
      mod_name = path.deconstantize
      mod = mod_name.empty? ? self : qualified_const_get(mod_name)
      mod.const_set(const_name, value)
    end

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.9 `parameterize`

The method `parameterize` normalizes its receiver in a way that can be used in pretty URLs.

    "John Smith".parameterize # => "john-smith"
    "Kurt Gödel".parameterize # => "kurt-godel"

In fact, the result string is wrapped in an instance of `ActiveSupport::Multibyte::Chars`.

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.10 `tableize`

The method `tableize` is `underscore` followed by `pluralize`.

    "Person".tableize      # => "people"
    "Invoice".tableize     # => "invoices"
    "InvoiceLine".tableize # => "invoice_lines"

As a rule of thumb, `tableize` returns the table name that corresponds to a given model for simple cases. The actual implementation in Active Record is not straight `tableize` indeed, because it also demodulizes the class name and checks a few options that may affect the returned string.

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.11 `classify`

The method `classify` is the inverse of `tableize`. It gives you the class name corresponding to a table name:

    "people".classify        # => "Person"
    "invoices".classify      # => "Invoice"
    "invoice_lines".classify # => "InvoiceLine"

The method understands qualified table names:

    "highrise_production.companies".classify # => "Company"

Note that `classify` returns a class name as a string. You can get the actual class object invoking `constantize` on it, explained next.

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.12 `constantize`

The method `constantize` resolves the constant reference expression in its receiver:

    "Fixnum".constantize # => Fixnum

    module M
      X = 1
    end
    "M::X".constantize # => 1

If the string evaluates to no known constant, or its content is not even a valid constant name, `constantize` raises `NameError`.

Constant name resolution by `constantize` starts always at the top-level `Object` even if there is no leading “::”.

    X = :in_Object
    module M
      X = :in_M

      X                 # => :in_M
      "::X".constantize # => :in_Object
      "X".constantize   # => :in_Object (!)
    end

So, it is in general not equivalent to what Ruby would do in the same spot, had a real constant be evaluated.

Mailer test cases obtain the mailer being tested from the name of the test class using `constantize`:

    # action_mailer/test_case.rb
    def determine_default_mailer(name)
      name.sub(/Test$/, '').constantize
    rescue NameError => e
      raise NonInferrableMailerError.new(name)
    end

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.13 `humanize`

The method `humanize` gives you a sensible name for display out of an attribute name. To do so it replaces underscores with spaces, removes any “_id” suffix, and capitalizes the first word:

    "name".humanize           # => "Name"
    "author_id".humanize      # => "Author"
    "comments_count".humanize # => "Comments count"

The helper method `full_messages` uses `humanize` as a fallback to include attribute names:

    def full_messages
      full_messages = []

      each do |attribute, messages|
        ...
        attr_name = attribute.to_s.gsub('.', '_').humanize
        attr_name = @base.class.human_attribute_name(attribute, :default => attr_name)
        ...
      end

      full_messages
    end

Defined in `active_support/core_ext/string/inflections.rb`.

##### 5.9.14 `foreign_key`

The method `foreign_key` gives a foreign key column name from a class name. To do so it demodulizes, underscores, and adds “_id”:

    "User".foreign_key           # => "user_id"
    "InvoiceLine".foreign_key    # => "invoice_line_id"
    "Admin::Session".foreign_key # => "session_id"

Pass a false argument if you do not want the underscore in “_id”:

    "User".foreign_key(false) # => "userid"

Associations use this method to infer foreign keys, for example `has_one` and `has_many` do this:

    # active_record/associations.rb
    foreign_key = options[:foreign_key] || reflection.active_record.name.foreign_key

Defined in `active_support/core_ext/string/inflections.rb`.

#### 5.10 Conversions

##### 5.10.1 `ord`

Ruby 1.9 defines `ord` to be the codepoint of the first character of the receiver. Active Support backports `ord` for single-byte encodings like ASCII or ISO-8859-1 in Ruby 1.8:

    "a".ord # => 97
    "à".ord # => 224, in ISO-8859-1

In Ruby 1.8 `ord` doesn’t work in general in UTF8 strings, use the multibyte support in Active Support for that:

    "a".mb_chars.ord # => 97
    "à".mb_chars.ord # => 224, in UTF8

Note that the 224 is different in both examples. In ISO-8859-1 “à” is represented as a single byte, 224. Its single-character representation in UTF8 has two bytes, namely 195 and 160, but its Unicode codepoint is 224. If we call `ord` on the UTF8 string “à” the return value will be 195 in Ruby 1.8. That is not an error, because UTF8 is unsupported, the call itself would be bogus.

`ord` is equivalent to `getbyte(0)`.

Defined in `active_support/core_ext/string/conversions.rb`.

##### 5.10.2 `getbyte`

Active Support backports `getbyte` from Ruby 1.9:

    "foo".getbyte(0)  # => 102, same as "foo".ord
    "foo".getbyte(1)  # => 111
    "foo".getbyte(9)  # => nil
    "foo".getbyte(-1) # => 111

`getbyte` is equivalent to `[]`.

Defined in `active_support/core_ext/string/conversions.rb`.

##### 5.10.3 `to_date`, `to_time`, `to_datetime`

The methods `to_date`, `to_time`, and `to_datetime` are basically convenience wrappers around `Date._parse`:

    "2010-07-27".to_date              # => Tue, 27 Jul 2010
    "2010-07-27 23:37:00".to_time     # => Tue Jul 27 23:37:00 UTC 2010
    "2010-07-27 23:37:00".to_datetime # => Tue, 27 Jul 2010 23:37:00 +0000

`to_time` receives an optional argument `:utc` or `:local`, to indicate which time zone you want the time in:

    "2010-07-27 23:42:00".to_time(:utc)   # => Tue Jul 27 23:42:00 UTC 2010
    "2010-07-27 23:42:00".to_time(:local) # => Tue Jul 27 23:42:00 +0200 2010

Default is `:utc`.

Please refer to the documentation of `Date._parse` for further details.

The three of them return `nil` for blank receivers.

Defined in `active_support/core_ext/string/conversions.rb`.

### 6 Extensions to `Numeric`

#### 6.1 Bytes

All numbers respond to these methods:

    bytes
    kilobytes
    megabytes
    gigabytes
    terabytes
    petabytes
    exabytes

They return the corresponding amount of bytes, using a conversion factor of 1024:

    2.kilobytes   # => 2048
    3.megabytes   # => 3145728
    3.5.gigabytes # => 3758096384
    -4.exabytes   # => -4611686018427387904

Singular forms are aliased so you are able to say:

    1.megabyte # => 1048576

Defined in `active_support/core_ext/numeric/bytes.rb`.

### 7 Extensions to `Integer`

#### 7.1 `multiple_of?`

The method `multiple_of?` tests whether an integer is multiple of the argument:

    2.multiple_of?(1) # => true
    1.multiple_of?(2) # => false

Defined in `active_support/core_ext/integer/multiple.rb`.

#### 7.2 `ordinalize`

The method `ordinalize` returns the ordinal string corresponding to the receiver integer:

    1.ordinalize    # => "1st"
    2.ordinalize    # => "2nd"
    53.ordinalize   # => "53rd"
    2009.ordinalize # => "2009th"
    -21.ordinalize  # => "-21st"
    -134.ordinalize # => "-134th"

Defined in `active_support/core_ext/integer/inflections.rb`.

### 8 Extensions to `Float`

#### 8.1 `round`

The built-in method `Float#round` rounds a float to the nearest integer. In Ruby 1.9 this method takes an optional argument to let you specify a precision. Active Support adds that functionality to `round` in previous versions of Ruby:

    Math::E.round(4) # => 2.7183

Defined in `active_support/core_ext/float/rounding.rb`.

### 9 Extensions to `BigDecimal`

…

### 10 Extensions to `Enumerable`

#### 10.1 `group_by`

Active Support redefines `group_by` in Ruby 1.8.7 so that it returns an ordered hash as in 1.9:

    entries_by_surname_initial = address_book.group_by do |entry|
      entry.surname.at(0).upcase
    end

Distinct block return values are added to the hash as they come, so that’s the resulting order.

Defined in `active_support/core_ext/enumerable.rb`.

#### 10.2 `sum`

The method `sum` adds the elements of an enumerable:

    [1, 2, 3].sum # => 6
    (1..100).sum  # => 5050

Addition only assumes the elements respond to `+`:

    [[1, 2], [2, 3], [3, 4]].sum    # => [1, 2, 2, 3, 3, 4]
    %w(foo bar baz).sum             # => "foobarbaz"
    {:a => 1, :b => 2, :c => 3}.sum # => [:b, 2, :c, 3, :a, 1]

The sum of an empty collection is zero by default, but this is customizable:

    [].sum    # => 0
    [].sum(1) # => 1

If a block is given, `sum` becomes an iterator that yields the elements of the collection and sums the returned values:

    (1..5).sum {|n| n * 2 } # => 30
    [2, 4, 6, 8, 10].sum    # => 30

The sum of an empty receiver can be customized in this form as well:

    [].sum(1) {|n| n**3} # => 1

The method `ActiveRecord::Observer#observed_subclasses` for example is implemented this way:

    def observed_subclasses
      observed_classes.sum([]) { |klass| klass.send(:subclasses) }
    end

Defined in `active_support/core_ext/enumerable.rb`.

#### 10.3 `each_with_object`

The `inject` method offers iteration with an accumulator:

    [2, 3, 4].inject(1) {|product, i| product*i } # => 24

The block is expected to return the value for the accumulator in the next iteration, and this makes building mutable objects a bit cumbersome:

    [1, 2].inject({}) {|h, i| h[i] = i**2; h} # => {1 => 1, 2 => 4}

See that spurious “`; h`”?

Active Support backports `each_with_object` from Ruby 1.9, which addresses that use case. It iterates over the collection, passes the accumulator, and returns the accumulator when done. You normally modify the accumulator in place. The example above would be written this way:

    [1, 2].each_with_object({}) {|i, h| h[i] = i**2} # => {1 => 1, 2 => 4}

Note that the item of the collection and the accumulator come in different order in `inject` and `each_with_object`.

Defined in `active_support/core_ext/enumerable.rb`.

#### 10.4 `index_by`

The method `index_by` generates a hash with the elements of an enumerable indexed by some key.

It iterates through the collection and passes each element to a block. The element will be keyed by the value returned by the block:

    invoices.index_by(&:number)
    # => {'2009-032' => <Invoice ...>, '2009-008' => <Invoice ...>, ...}

Keys should normally be unique. If the block returns the same value for different elements no collection is built for that key. The last item will win.

Defined in `active_support/core_ext/enumerable.rb`.

#### 10.5 `many?`

The method `many?` is shorthand for `collection.size > 1`:

    <% if pages.many? %>
      <%= pagination_links %>
    <% end %>

If an optional block is given, `many?` only takes into account those elements that return true:

    @see_more = videos.many? {|video| video.category == params[:category]}

Defined in `active_support/core_ext/enumerable.rb`.

#### 10.6 `exclude?`

The predicate `exclude?` tests whether a given object does not belong to the collection. It is the negation of the built-in `include?`:

    to_visit << node if visited.exclude?(node)

Defined in `active_support/core_ext/enumerable.rb`.

### 11 Extensions to `Array`

#### 11.1 Accessing

Active Support augments the API of arrays to ease certain ways of accessing them. For example, `to` returns the subarray of elements up to the one at the passed index:

    %w(a b c d).to(2) # => %w(a b c)
    [].to(7)          # => []

Similarly, `from` returns the tail from the element at the passed index to the end. If the index is greater than the length of the array, it returns an empty array.

    %w(a b c d).from(2)  # => %w(c d)
    %w(a b c d).from(10) # => []
    [].from(0)           # => []

The methods `second`, `third`, `fourth`, and `fifth` return the corresponding element (`first` is built-in). Thanks to social wisdom and positive constructiveness all around, `forty_two` is also available.

    %w(a b c d).third # => c
    %w(a b c d).fifth # => nil

Defined in `active_support/core_ext/array/access.rb`.

#### 11.2 Random Access

Active Support backports `sample` from Ruby 1.9:

    shape_type = [Circle, Square, Triangle].sample
    # => Square, for example

    shape_types = [Circle, Square, Triangle].sample(2)
    # => [Triangle, Circle], for example

Defined in `active_support/core_ext/array/random_access.rb`.

#### 11.3 Adding Elements

##### 11.3.1 `prepend`

This method is an alias of `Array#unshift`.

    %w(a b c d).prepend('e')  # => %w(e a b c d)
    [].prepend(10)            # => [10]

Defined in `active_support/core_ext/array/prepend_and_append.rb`.

##### 11.3.2 `append`

This method is an alias of `Array#<<`.

    %w(a b c d).append('e')  # => %w(a b c d e)
    [].append([1,2])         # => [[1,2]]

Defined in `active_support/core_ext/array/prepend_and_append.rb`.

#### 11.4 Options Extraction

When the last argument in a method call is a hash, except perhaps for a `&block` argument, Ruby allows you to omit the brackets:

    User.exists?(:email => params[:email])

That syntactic sugar is used a lot in Rails to avoid positional arguments where there would be too many, offering instead interfaces that emulate named parameters. In particular it is very idiomatic to use a trailing hash for options.

If a method expects a variable number of arguments and uses `*` in its declaration, however, such an options hash ends up being an item of the array of arguments, where it loses its role.

In those cases, you may give an options hash a distinguished treatment with `extract_options!`. This method checks the type of the last item of an array. If it is a hash it pops it and returns it, otherwise it returns an empty hash.

Let’s see for example the definition of the `caches_action` controller macro:

    def caches_action(*actions)
      return unless cache_configured?
      options = actions.extract_options!
      ...
    end

This method receives an arbitrary number of action names, and an optional hash of options as last argument. With the call to `extract_options!` you obtain the options hash and remove it from `actions` in a simple and explicit way.

Defined in `active_support/core_ext/array/extract_options.rb`.

#### 11.5 Conversions

##### 11.5.1 `to_sentence`

The method `to_sentence` turns an array into a string containing a sentence that enumerates its items:

    %w().to_sentence                # => ""
    %w(Earth).to_sentence           # => "Earth"
    %w(Earth Wind).to_sentence      # => "Earth and Wind"
    %w(Earth Wind Fire).to_sentence # => "Earth, Wind, and Fire"

This method accepts three options:

- `:two_words_connector`: What is used for arrays of length 2. Default is " and ".
- `:words_connector`: What is used to join the elements of arrays with 3 or more elements, except for the last two. Default is ", ".
- `:last_word_connector`: What is used to join the last items of an array with 3 or more elements. Default is ", and ".

The defaults for these options can be localised, their keys are:

Option | I18n key
--- | ---
`:two_words_connector` | `support.array.two_words_connector`
`:words_connector` | `support.array.words_connector`
`:last_word_connector` | `support.array.last_word_connector`

Options `:connector` and `:skip_last_comma` are deprecated.

Defined in `active_support/core_ext/array/conversions.rb`.

##### 11.5.2 `to_formatted_s`

The method `to_formatted_s` acts like `to_s` by default.

If the array contains items that respond to `id`, however, it may be passed the symbol `:db` as argument. That’s typically used with collections of ARs, though technically any object in Ruby 1.8 responds to `id` indeed. Returned strings are:

    [].to_formatted_s(:db)            # => "null"
    [user].to_formatted_s(:db)        # => "8456"
    invoice.lines.to_formatted_s(:db) # => "23,567,556,12"

Integers in the example above are supposed to come from the respective calls to `id`.

Defined in `active_support/core_ext/array/conversions.rb`.

##### 11.5.3 `to_xml`

The method `to_xml` returns a string containing an XML representation of its receiver:

    Contributor.limit(2).order(:rank).to_xml
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <contributors type="array">
    #   <contributor>
    #     <id type="integer">4356</id>
    #     <name>Jeremy Kemper</name>
    #     <rank type="integer">1</rank>
    #     <url-id>jeremy-kemper</url-id>
    #   </contributor>
    #   <contributor>
    #     <id type="integer">4404</id>
    #     <name>David Heinemeier Hansson</name>
    #     <rank type="integer">2</rank>
    #     <url-id>david-heinemeier-hansson</url-id>
    #   </contributor>
    # </contributors>

To do so it sends `to_xml` to every item in turn, and collects the results under a root node. All items must respond to `to_xml`, an exception is raised otherwise.

By default, the name of the root element is the underscorized and dasherized plural of the name of the class of the first item, provided the rest of elements belong to that type (checked with `is_a?`) and they are not hashes. In the example above that’s “contributors”.

If there’s any element that does not belong to the type of the first one the root node becomes “records”:

    [Contributor.first, Commit.first].to_xml
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <records type="array">
    #   <record>
    #     <id type="integer">4583</id>
    #     <name>Aaron Batalion</name>
    #     <rank type="integer">53</rank>
    #     <url-id>aaron-batalion</url-id>
    #   </record>
    #   <record>
    #     <author>Joshua Peek</author>
    #     <authored-timestamp type="datetime">2009-09-02T16:44:36Z</authored-timestamp>
    #     <branch>origin/master</branch>
    #     <committed-timestamp type="datetime">2009-09-02T16:44:36Z</committed-timestamp>
    #     <committer>Joshua Peek</committer>
    #     <git-show nil="true"></git-show>
    #     <id type="integer">190316</id>
    #     <imported-from-svn type="boolean">false</imported-from-svn>
    #     <message>Kill AMo observing wrap_with_notifications since ARes was only using it</message>
    #     <sha1>723a47bfb3708f968821bc969a9a3fc873a3ed58</sha1>
    #   </record>
    # </records>

If the receiver is an array of hashes the root element is by default also “records”:

    [{:a => 1, :b => 2}, {:c => 3}].to_xml
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <records type="array">
    #   <record>
    #     <b type="integer">2</b>
    #     <a type="integer">1</a>
    #   </record>
    #   <record>
    #     <c type="integer">3</c>
    #   </record>
    # </records>

If the collection is empty the root element is by default “nil-classes”. That’s a gotcha, for example the root element of the list of contributors above would not be “contributors” if the collection was empty, but “nil-classes”. You may use the `:root` option to ensure a consistent root element.

The name of children nodes is by default the name of the root node singularized. In the examples above we’ve seen “contributor” and “record”. The option `:children` allows you to set these node names.

The default XML builder is a fresh instance of `Builder::XmlMarkup`. You can configure your own builder via the `:builder` option. The method also accepts options like `:dasherize` and friends, they are forwarded to the builder:

    Contributor.limit(2).order(:rank).to_xml(:skip_types => true)
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <contributors>
    #   <contributor>
    #     <id>4356</id>
    #     <name>Jeremy Kemper</name>
    #     <rank>1</rank>
    #     <url-id>jeremy-kemper</url-id>
    #   </contributor>
    #   <contributor>
    #     <id>4404</id>
    #     <name>David Heinemeier Hansson</name>
    #     <rank>2</rank>
    #     <url-id>david-heinemeier-hansson</url-id>
    #   </contributor>
    # </contributors>

Defined in `active_support/core_ext/array/conversions.rb`.

#### 11.6 Wrapping

The method `Array.wrap` wraps its argument in an array unless it is already an array (or array-like).

Specifically:

- If the argument is `nil` an empty list is returned.
- Otherwise, if the argument responds to `to_ary` it is invoked, and if the value of `to_ary` is not `nil`, it is returned.
- Otherwise, an array with the argument as its single element is returned.

    Array.wrap(nil)       # => []
    Array.wrap([1, 2, 3]) # => [1, 2, 3]
    Array.wrap(0)         # => [0]

This method is similar in purpose to `Kernel#Array`, but there are some differences:

- If the argument responds to `to_ary` the method is invoked. `Kernel#Array` moves on to try `to_a` if the returned value is `nil`, but `Array.wrap` returns `nil` right away.
- If the returned value from `to_ary` is neither `nil` nor an `Array` object, `Kernel#Array` raises an exception, while `Array.wrap` does not, it just returns the value.
- It does not call `to_a` on the argument, though special-cases `nil` to return an empty array.

The last point is particularly worth comparing for some enumerables:

    Array.wrap(:foo => :bar) # => [{:foo => :bar}]
    Array(:foo => :bar)      # => [[:foo, :bar]]

    Array.wrap("foo\nbar")   # => ["foo\nbar"]
    Array("foo\nbar")        # => ["foo\n", "bar"], in Ruby 1.8

There’s also a related idiom that uses the splat operator:

    [*object]

which in Ruby 1.8 returns `[nil]` for `nil`, and calls to `Array(object)` otherwise. (Please if you know the exact behavior in 1.9 contact fxn.)

Thus, in this case the behavior is different for `nil`, and the differences with `Kernel#Array` explained above apply to the rest of `object`s.

Defined in `active_support/core_ext/array/wrap.rb`.

#### 11.7 Grouping

##### 11.7.1 `in_groups_of(number, fill_with = nil)`

The method `in_groups_of` splits an array into consecutive groups of a certain size. It returns an array with the groups:

    [1, 2, 3].in_groups_of(2) # => [[1, 2], [3, nil]]

or yields them in turn if a block is passed:

    <% sample.in_groups_of(3) do |a, b, c| %>
      <tr>
        <td><%=h a %></td>
        <td><%=h b %></td>
        <td><%=h c %></td>
      </tr>
    <% end %>

The first example shows `in_groups_of` fills the last group with as many `nil` elements as needed to have the requested size. You can change this padding value using the second optional argument:

    [1, 2, 3].in_groups_of(2, 0) # => [[1, 2], [3, 0]]

And you can tell the method not to fill the last group passing `false`:

    [1, 2, 3].in_groups_of(2, false) # => [[1, 2], [3]]

As a consequence `false` can’t be a used as a padding value.

Defined in `active_support/core_ext/array/grouping.rb`.

##### 11.7.2 `in_groups(number, fill_with = nil)`

The method `in_groups` splits an array into a certain number of groups. The method returns an array with the groups:

    %w(1 2 3 4 5 6 7).in_groups(3)
    # => [["1", "2", "3"], ["4", "5", nil], ["6", "7", nil]]

or yields them in turn if a block is passed:

    %w(1 2 3 4 5 6 7).in_groups(3) {|group| p group}
    ["1", "2", "3"]
    ["4", "5", nil]
    ["6", "7", nil]

The examples above show that `in_groups` fills some groups with a trailing `nil` element as needed. A group can get at most one of these extra elements, the rightmost one if any. And the groups that have them are always the last ones.

You can change this padding value using the second optional argument:

    %w(1 2 3 4 5 6 7).in_groups(3, "0")
    # => [["1", "2", "3"], ["4", "5", "0"], ["6", "7", "0"]]

And you can tell the method not to fill the smaller groups passing `false`:

    %w(1 2 3 4 5 6 7).in_groups(3, false)
    # => [["1", "2", "3"], ["4", "5"], ["6", "7"]]

As a consequence `false` can’t be a used as a padding value.

Defined in `active_support/core_ext/array/grouping.rb`.

##### 11.7.3 `split(value = nil)`

The method `split` divides an array by a separator and returns the resulting chunks.

If a block is passed the separators are those elements of the array for which the block returns true:

    (-5..5).to_a.split { |i| i.multiple_of?(4) }
    # => [[-5], [-3, -2, -1], [1, 2, 3], [5]]

Otherwise, the value received as argument, which defaults to `nil`, is the separator:

    [0, 1, -5, 1, 1, "foo", "bar"].split(1)
    # => [[0], [-5], [], ["foo", "bar"]]

Observe in the previous example that consecutive separators result in empty arrays.

Defined in `active_support/core_ext/array/grouping.rb`.

### 12 Extensions to `Hash`

#### 12.1 Conversions

##### 12.1.1 `to_xml`

The method `to_xml` returns a string containing an XML representation of its receiver:

    {"foo" => 1, "bar" => 2}.to_xml
    # =>
    # <?xml version="1.0" encoding="UTF-8"?>
    # <hash>
    #   <foo type="integer">1</foo>
    #   <bar type="integer">2</bar>
    # </hash>

To do so, the method loops over the pairs and builds nodes that depend on the values. Given a pair `key`, `value`:

- If `value` is a hash there’s a recursive call with `key` as `:root`.

- If `value` is an array there’s a recursive call with `key` as `:root`, and `key` singularized as `:children`.

- If `value` is a callable object it must expect one or two arguments. Depending on the arity, the callable is invoked with the `options` hash as first argument with `key` as `:root`, and `key` singularized as second argument. Its return value becomes a new node.

- If `value` responds to `to_xml` the method is invoked with `key` as `:root`.

- Otherwise, a node with `key` as tag is created with a string representation of `value` as text node. If `value` is `nil` an attribute “nil” set to “true” is added. Unless the option `:skip_types` exists and is true, an attribute “type” is added as well according to the following mapping:

      XML_TYPE_NAMES = {
        "Symbol"     => "symbol",
        "Fixnum"     => "integer",
        "Bignum"     => "integer",
        "BigDecimal" => "decimal",
        "Float"      => "float",
        "TrueClass"  => "boolean",
        "FalseClass" => "boolean",
        "Date"       => "date",
        "DateTime"   => "datetime",
        "Time"       => "datetime"
      }

By default the root node is “hash”, but that’s configurable via the `:root` option.

The default XML builder is a fresh instance of `Builder::XmlMarkup`. You can configure your own builder with the `:builder` option. The method also accepts options like `:dasherize` and friends, they are forwarded to the builder.

Defined in `active_support/core_ext/hash/conversions.rb`.

#### 12.2 Merging

Ruby has a built-in method `Hash#merge` that merges two hashes:

    {:a => 1, :b => 1}.merge(:a => 0, :c => 2)
    # => {:a => 0, :b => 1, :c => 2}

Active Support defines a few more ways of merging hashes that may be convenient.

##### 12.2.1 `reverse_merge` and `reverse_merge!`

In case of collision the key in the hash of the argument wins in `merge`. You can support option hashes with default values in a compact way with this idiom:

    options = {:length => 30, :omission => "..."}.merge(options)

Active Support defines `reverse_merge` in case you prefer this alternative notation:

    options = options.reverse_merge(:length => 30, :omission => "...")

And a bang version `reverse_merge!` that performs the merge in place:

    options.reverse_merge!(:length => 30, :omission => "...")

Take into account that `reverse_merge!` may change the hash in the caller, which may or may not be a good idea.

Defined in `active_support/core_ext/hash/reverse_merge.rb`.

##### 12.2.2 `reverse_update`

The method `reverse_update` is an alias for `reverse_merge!`, explained above.

Note that `reverse_update` has no bang.

Defined in `active_support/core_ext/hash/reverse_merge.rb`.

##### 12.2.3 `deep_merge` and `deep_merge!`

As you can see in the previous example if a key is found in both hashes the value in the one in the argument wins.

Active Support defines `Hash#deep_merge`. In a deep merge, if a key is found in both hashes and their values are hashes in turn, then their merge becomes the value in the resulting hash:

    {:a => {:b => 1}}.deep_merge(:a => {:c => 2})
    # => {:a => {:b => 1, :c => 2}}

The method `deep_merge!` performs a deep merge in place.

Defined in `active_support/core_ext/hash/deep_merge.rb`.

#### 12.3 Diffing

The method `diff` returns a hash that represents a diff of the receiver and the argument with the following logic:

- Pairs `key`, `value` that exist in both hashes do not belong to the diff hash.

- If both hashes have `key`, but with different values, the pair in the receiver wins.

- The rest is just merged.

    {:a => 1}.diff(:a => 1)
    # => {}, first rule

    {:a => 1}.diff(:a => 2)
    # => {:a => 1}, second rule

    {:a => 1}.diff(:b => 2)
    # => {:a => 1, :b => 2}, third rule

    {:a => 1, :b => 2, :c => 3}.diff(:b => 1, :c => 3, :d => 4)
    # => {:a => 1, :b => 2, :d => 4}, all rules

    {}.diff({})        # => {}
    {:a => 1}.diff({}) # => {:a => 1}
    {}.diff(:a => 1)   # => {:a => 1}

An important property of this diff hash is that you can retrieve the original hash by applying `diff` twice:

    hash.diff(hash2).diff(hash2) == hash

Diffing hashes may be useful for error messages related to expected option hashes for example.

Defined in `active_support/core_ext/hash/diff.rb`.

#### 12.4 Working with Keys

##### 12.4.1 `except` and `except!`

The method `except` returns a hash with the keys in the argument list removed, if present:

    {:a => 1, :b => 2}.except(:a) # => {:b => 2}

If the receiver responds to `convert_key`, the method is called on each of the arguments. This allows `except` to play nice with hashes with indifferent access for instance:

    {:a => 1}.with_indifferent_access.except(:a)  # => {}
    {:a => 1}.with_indifferent_access.except("a") # => {}

The method `except` may come in handy for example when you want to protect some parameter that can’t be globally protected with `attr_protected`:

    params[:account] = params[:account].except(:plan_id) unless admin?
    @account.update_attributes(params[:account])

There’s also the bang variant `except!` that removes keys in the very receiver.

Defined in `active_support/core_ext/hash/except.rb`.

##### 12.4.2 `stringify_keys` and `stringify_keys!`

The method `stringify_keys` returns a hash that has a stringified version of the keys in the receiver. It does so by sending `to_s` to them:

    {nil => nil, 1 => 1, :a => :a}.stringify_keys
    # => {"" => nil, "a" => :a, "1" => 1}

The result in case of collision is undefined:

    {"a" => 1, :a => 2}.stringify_keys
    # => {"a" => 2}, in my test, can't rely on this result though

This method may be useful for example to easily accept both symbols and strings as options. For instance `ActionView::Helpers::FormHelper` defines:

    def to_check_box_tag(options = {}, checked_value = "1", unchecked_value = "0")
      options = options.stringify_keys
      options["type"] = "checkbox"
      ...
    end

The second line can safely access the “type” key, and let the user to pass either `:type` or “type”.

There’s also the bang variant `stringify_keys!` that stringifies keys in the very receiver.

Defined in `active_support/core_ext/hash/keys.rb`.

##### 12.4.3 `symbolize_keys` and `symbolize_keys!`

The method `symbolize_keys` returns a hash that has a symbolized version of the keys in the receiver, where possible. It does so by sending `to_sym` to them:

    {nil => nil, 1 => 1, "a" => "a"}.symbolize_keys
    # => {1 => 1, nil => nil, :a => "a"}

Note in the previous example only one key was symbolized.

The result in case of collision is undefined:

    {"a" => 1, :a => 2}.symbolize_keys
    # => {:a => 2}, in my test, can't rely on this result though

This method may be useful for example to easily accept both symbols and strings as options. For instance `ActionController::UrlRewriter` defines

    def rewrite_path(options)
      options = options.symbolize_keys
      options.update(options[:params].symbolize_keys) if options[:params]
      ...
    end

The second line can safely access the `:params` key, and let the user to pass either `:params` or “params”.

There’s also the bang variant `symbolize_keys!` that symbolizes keys in the very receiver.

Defined in `active_support/core_ext/hash/keys.rb`.

##### 12.4.4 `to_options` and `to_options!`

The methods `to_options` and `to_options!` are respectively aliases of `symbolize_keys` and `symbolize_keys!`.

Defined in `active_support/core_ext/hash/keys.rb`.

##### 12.4.5 `assert_valid_keys`

The method `assert_valid_keys` receives an arbitrary number of arguments, and checks whether the receiver has any key outside that white list. If it does `ArgumentError` is raised.

    {:a => 1}.assert_valid_keys(:a)  # passes
    {:a => 1}.assert_valid_keys("a") # ArgumentError

Active Record does not accept unknown options when building associations for example. It implements that control via `assert_valid_keys`:

    mattr_accessor :valid_keys_for_has_many_association
    @@valid_keys_for_has_many_association = [
      :class_name, :table_name, :foreign_key, :primary_key,
      :dependent,
      :select, :conditions, :include, :order, :group, :having, :limit, :offset,
      :as, :through, :source, :source_type,
      :uniq,
      :finder_sql, :counter_sql,
      :before_add, :after_add, :before_remove, :after_remove,
      :extend, :readonly,
      :validate, :inverse_of
    ]

    def create_has_many_reflection(association_id, options, &extension)
      options.assert_valid_keys(valid_keys_for_has_many_association)
      ...
    end

Defined in `active_support/core_ext/hash/keys.rb`.

#### 12.5 Slicing

Ruby has built-in support for taking slices out of strings and arrays. Active Support extends slicing to hashes:

    {:a => 1, :b => 2, :c => 3}.slice(:a, :c)
    # => {:c => 3, :a => 1}

    {:a => 1, :b => 2, :c => 3}.slice(:b, :X)
    # => {:b => 2} # non-existing keys are ignored

If the receiver responds to `convert_key` keys are normalized:

    {:a => 1, :b => 2}.with_indifferent_access.slice("a")
    # => {:a => 1}

Slicing may come in handy for sanitizing option hashes with a white list of keys.

There’s also `slice!` which in addition to perform a slice in place returns what’s removed:

    hash = {:a => 1, :b => 2}
    rest = hash.slice!(:a) # => {:b => 2}
    hash                   # => {:a => 1}

Defined in `active_support/core_ext/hash/slice.rb`.

#### 12.6 Extracting

The method `extract!` removes and returns the key/value pairs matching the given keys.

    hash = {:a => 1, :b => 2}
    rest = hash.extract!(:a) # => {:a => 1}
    hash                     # => {:b => 2}

Defined in `active_support/core_ext/hash/slice.rb`.

#### 12.7 Indifferent Access

The method `with_indifferent_access` returns an `ActiveSupport::HashWithIndifferentAccess` out of its receiver:

    {:a => 1}.with_indifferent_access["a"] # => 1

Defined in `active_support/core_ext/hash/indifferent_access.rb`.

### 13 Extensions to `Regexp`

#### 13.1 `multiline?`

The method `multiline?` says whether a regexp has the `/m` flag set, that is, whether the dot matches newlines.

    %r{.}.multiline?  # => false
    %r{.}m.multiline? # => true

    Regexp.new('.').multiline?                    # => false
    Regexp.new('.', Regexp::MULTILINE).multiline? # => true

Rails uses this method in a single place, also in the routing code. Multiline regexps are disallowed for route requirements and this flag eases enforcing that constraint.

    def assign_route_options(segments, defaults, requirements)
      ...
      if requirement.multiline?
        raise ArgumentError, "Regexp multiline option not allowed in routing requirements: #{requirement.inspect}"
      end
      ...
    end

Defined in `active_support/core_ext/regexp.rb`.

### 14 Extensions to `Range`

#### 14.1 `to_s`

Active Support extends the method `Range#to_s` so that it understands an optional format argument. As of this writing the only supported non-default format is `:db`:

    (Date.today..Date.tomorrow).to_s
    # => "2009-10-25..2009-10-26"

    (Date.today..Date.tomorrow).to_s(:db)
    # => "BETWEEN '2009-10-25' AND '2009-10-26'"

As the example depicts, the `:db` format generates a `BETWEEN` SQL clause. That is used by Active Record in its support for range values in conditions.

Defined in `active_support/core_ext/range/conversions.rb`.

#### 14.2 `step`

Active Support extends the method `Range#step` so that it can be invoked without a block:

    (1..10).step(2) # => [1, 3, 5, 7, 9]

As the example shows, in that case the method returns an array with the corresponding elements.

Defined in `active_support/core_ext/range/blockless_step.rb`.

#### 14.3 `include?`

The method `Range#include?` says whether some value falls between the ends of a given instance:

    (2..3).include?(Math::E) # => true

Active Support extends this method so that the argument may be another range in turn. In that case we test whether the ends of the argument range belong to the receiver themselves:

    (1..10).include?(3..7)  # => true
    (1..10).include?(0..7)  # => false
    (1..10).include?(3..11) # => false
    (1...9).include?(3..9)  # => false

The original `Range#include?` is still the one aliased to `Range#===`.

Defined in `active_support/core_ext/range/include_range.rb`.

#### 14.4 `cover?`

Ruby 1.9 provides `cover?`, and Active Support defines it for previous versions as an alias for `include?`.

The method `include?` in Ruby 1.9 is different from the one in 1.8 for non-numeric ranges: instead of being based on comparisons between the value and the range’s endpoints, it walks the range with `succ` looking for value. This works better for ranges with holes, but it has different complexity and may not finish in some other cases.

In Ruby 1.9 the old behavior is still available in the new `cover?`, which Active Support backports for forward compatibility. For example, Rails uses `cover?` for ranges in `validates_inclusion_of`.

#### 14.5 `overlaps?`

The method `Range#overlaps?` says whether any two given ranges have non-void intersection:

    (1..10).overlaps?(7..11)  # => true
    (1..10).overlaps?(0..7)   # => true
    (1..10).overlaps?(11..27) # => false

Defined in `active_support/core_ext/range/overlaps.rb`.

### 15 Extensions to `Proc`

#### 15.1 `bind`

As you surely know Ruby has an `UnboundMethod` class whose instances are methods that belong to the limbo of methods without a self. The method `Module#instance_method` returns an unbound method for example:

    Hash.instance_method(:delete) # => #<UnboundMethod: Hash#delete>

An unbound method is not callable as is, you need to bind it first to an object with `bind`:

    clear = Hash.instance_method(:clear)
    clear.bind({:a => 1}).call # => {}

Active Support defines `Proc#bind` with an analogous purpose:

    Proc.new { size }.bind([]).call # => 0

As you see that’s callable and bound to the argument, the return value is indeed a `Method`.

To do so `Proc#bind` actually creates a method under the hood. If you ever see a method with a weird name like `__bind_1256598120_237302` in a stack trace you know now where it comes from.

Action Pack uses this trick in `rescue_from` for example, which accepts the name of a method and also a proc as callbacks for a given rescued exception. It has to call them in either case, so a bound method is returned by `handler_for_rescue`, thus simplifying the code in the caller:

    def handler_for_rescue(exception)
      _, rescuer = Array(rescue_handlers).reverse.detect do |klass_name, handler|
        ...
      end

      case rescuer
      when Symbol
        method(rescuer)
      when Proc
        rescuer.bind(self)
      end
    end

Defined in `active_support/core_ext/proc.rb`.

### 16 Extensions to `Date`

#### 16.1 Calculations

All the following methods are defined in `active_support/core_ext/date/calculations.rb`.

The following calculation methods have edge cases in October 1582, since days 5..14 just do not exist. This guide does not document their behavior around those days for brevity, but it is enough to say that they do what you would expect. That is, `Date.new(1582, 10, 4).tomorrow` returns `Date.new(1582, 10, 15)` and so on. Please check `test/core_ext/date_ext_test.rb` in the Active Support test suite for expected behavior.

##### 16.1.1 `Date.current`

Active Support defines `Date.current` to be today in the current time zone. That’s like `Date.today`, except that it honors the user time zone, if defined. It also defines `Date.yesterday` and `Date.tomorrow`, and the instance predicates `past?`, `today?`, and `future?`, all of them relative to `Date.current`.

When making Date comparisons using methods which honor the user time zone, make sure to use `Date.current` and not `Date.today`. There are cases where the user time zone might be in the future compared to the system time zone, which `Date.today` uses by default. This means `Date.today` may equal `Date.yesterday`.

##### 16.1.2 Named dates

###### 16.1.2.1 `prev_year`, `next_year`

In Ruby 1.9 `prev_year` and `next_year` return a date with the same day/month in the last or next year:

    d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
    d.prev_year              # => Fri, 08 May 2009
    d.next_year              # => Sun, 08 May 2011

If date is the 29th of February of a leap year, you obtain the 28th:

    d = Date.new(2000, 2, 29) # => Tue, 29 Feb 2000
    d.prev_year               # => Sun, 28 Feb 1999
    d.next_year               # => Wed, 28 Feb 2001

Active Support defines these methods as well for Ruby 1.8.

###### 16.1.2.2 `prev_month`, `next_month`

In Ruby 1.9 `prev_month` and `next_month` return the date with the same day in the last or next month:

    d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
    d.prev_month             # => Thu, 08 Apr 2010
    d.next_month             # => Tue, 08 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:

    Date.new(2000, 5, 31).prev_month # => Sun, 30 Apr 2000
    Date.new(2000, 3, 31).prev_month # => Tue, 29 Feb 2000
    Date.new(2000, 5, 31).next_month # => Fri, 30 Jun 2000
    Date.new(2000, 1, 31).next_month # => Tue, 29 Feb 2000

Active Support defines these methods as well for Ruby 1.8.

###### 16.1.2.3 `beginning_of_week`, `end_of_week`

The methods `beginning_of_week` and `end_of_week` return the dates for the beginning and end of week, assuming weeks start on Monday:

    d = Date.new(2010, 5, 8) # => Sat, 08 May 2010
    d.beginning_of_week      # => Mon, 03 May 2010
    d.end_of_week            # => Sun, 09 May 2010

`beginning_of_week` is aliased to `monday` and `at_beginning_of_week`. `end_of_week` is aliased to `sunday` and `at_end_of_week`.

###### 16.1.2.4 `prev_week`, `next_week`

The method `next_week` receives a symbol with a day name in English (in lowercase, default is `:monday`) and it returns the date corresponding to that day:

    d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
    d.next_week              # => Mon, 10 May 2010
    d.next_week(:saturday)   # => Sat, 15 May 2010

The method `prev_week` is analogous:

    d.prev_week              # => Mon, 26 Apr 2010
    d.prev_week(:saturday)   # => Sat, 01 May 2010
    d.prev_week(:friday)     # => Fri, 30 Apr 2010

###### 16.1.2.5 `beginning_of_month`, `end_of_month`

The methods `beginning_of_month` and `end_of_month` return the dates for the beginning and end of the month:

    d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
    d.beginning_of_month     # => Sat, 01 May 2010
    d.end_of_month           # => Mon, 31 May 2010

`beginning_of_month` is aliased to `at_beginning_of_month`, and `end_of_month` is aliased to `at_end_of_month`.

###### 16.1.2.6 `beginning_of_quarter`, `end_of_quarter`

The methods `beginning_of_quarter` and `end_of_quarter` return the dates for the beginning and end of the quarter of the receiver’s calendar year:

    d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
    d.beginning_of_quarter   # => Thu, 01 Apr 2010
    d.end_of_quarter         # => Wed, 30 Jun 2010

`beginning_of_quarter` is aliased to `at_beginning_of_quarter`, and `end_of_quarter` is aliased to `at_end_of_quarter`.

###### 16.1.2.7 `beginning_of_year`, `end_of_year`

The methods `beginning_of_year` and `end_of_year` return the dates for the beginning and end of the year:

    d = Date.new(2010, 5, 9) # => Sun, 09 May 2010
    d.beginning_of_year      # => Fri, 01 Jan 2010
    d.end_of_year            # => Fri, 31 Dec 2010

`beginning_of_year` is aliased to `at_beginning_of_year`, and `end_of_year` is aliased to `at_end_of_year`.

##### 16.1.3 Other Date Computations

###### 16.1.3.1 `years_ago`, `years_since`

The method `years_ago` receives a number of years and returns the same date those many years ago:

    date = Date.new(2010, 6, 7)
    date.years_ago(10) # => Wed, 07 Jun 2000

`years_since` moves forward in time:

    date = Date.new(2010, 6, 7)
    date.years_since(10) # => Sun, 07 Jun 2020

If such a day does not exist, the last day of the corresponding month is returned:

    Date.new(2012, 2, 29).years_ago(3)     # => Sat, 28 Feb 2009
    Date.new(2012, 2, 29).years_since(3)   # => Sat, 28 Feb 2015

###### 16.1.3.2 `months_ago`, `months_since`

The methods `months_ago` and `months_since` work analogously for months:

    Date.new(2010, 4, 30).months_ago(2)   # => Sun, 28 Feb 2010
    Date.new(2010, 4, 30).months_since(2) # => Wed, 30 Jun 2010

If such a day does not exist, the last day of the corresponding month is returned:

    Date.new(2010, 4, 30).months_ago(2)    # => Sun, 28 Feb 2010
    Date.new(2009, 12, 31).months_since(2) # => Sun, 28 Feb 2010

###### 16.1.3.3 `weeks_ago`

The method `weeks_ago` works analogously for weeks:

    Date.new(2010, 5, 24).weeks_ago(1)    # => Mon, 17 May 2010
    Date.new(2010, 5, 24).weeks_ago(2)    # => Mon, 10 May 2010

###### 16.1.3.4 `advance`

The most generic way to jump to other days is `advance`. This method receives a hash with keys `:years`, `:months`, `:weeks`, `:days`, and returns a date advanced as much as the present keys indicate:

    date = Date.new(2010, 6, 6)
    date.advance(:years => 1, :weeks => 2)  # => Mon, 20 Jun 2011
    date.advance(:months => 2, :days => -2) # => Wed, 04 Aug 2010

Note in the previous example that increments may be negative.

To perform the computation the method first increments years, then months, then weeks, and finally days. This order is important towards the end of months. Say for example we are at the end of February of 2010, and we want to move one month and one day forward.

The method `advance` advances first one month, and then one day, the result is:

    Date.new(2010, 2, 28).advance(:months => 1, :days => 1)
    # => Sun, 29 Mar 2010

While if it did it the other way around the result would be different:

    Date.new(2010, 2, 28).advance(:days => 1).advance(:months => 1)
    # => Thu, 01 Apr 2010

##### 16.1.4 Changing Components

The method `change` allows you to get a new date which is the same as the receiver except for the given year, month, or day:

    Date.new(2010, 12, 23).change(:year => 2011, :month => 11)
    # => Wed, 23 Nov 2011

This method is not tolerant to non-existing dates, if the change is invalid `ArgumentError` is raised:

    Date.new(2010, 1, 31).change(:month => 2)
    # => ArgumentError: invalid date

##### 16.1.5 Durations

Durations can be added to and subtracted from dates:

    d = Date.current
    # => Mon, 09 Aug 2010
    d + 1.year
    # => Tue, 09 Aug 2011
    d - 3.hours
    # => Sun, 08 Aug 2010 21:00:00 UTC +00:00

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

    Date.new(1582, 10, 4) + 1.day
    # => Fri, 15 Oct 1582

##### 16.1.6 Timestamps

The following methods return a `Time` object if possible, otherwise a `DateTime`. If set, they honor the user time zone.

###### 16.1.6.1 `beginning_of_day`, `end_of_day`

The method `beginning_of_day` returns a timestamp at the beginning of the day (00:00:00):

    date = Date.new(2010, 6, 7)
    date.beginning_of_day # => Sun Jun 07 00:00:00 +0200 2010

The method `end_of_day` returns a timestamp at the end of the day (23:59:59):

    date = Date.new(2010, 6, 7)
    date.end_of_day # => Sun Jun 06 23:59:59 +0200 2010

`beginning_of_day` is aliased to `at_beginning_of_day`, `midnight`, `at_midnight`.

###### 16.1.6.2 `ago`, `since`

The method `ago` receives a number of seconds as argument and returns a timestamp those many seconds ago from midnight:

    date = Date.current # => Fri, 11 Jun 2010
    date.ago(1)         # => Thu, 10 Jun 2010 23:59:59 EDT -04:00

Similarly, `since` moves forward:

    date = Date.current # => Fri, 11 Jun 2010
    date.since(1)       # => Fri, 11 Jun 2010 00:00:01 EDT -04:00

##### 16.1.7 Other Time Computations

#### 16.2 Conversions

### 17 Extensions to `DateTime`

`DateTime` is not aware of DST rules and so some of these methods have edge cases when a DST change is going on. For example `seconds_since_midnight` might not return the real amount in such a day.

#### 17.1 Calculations

All the following methods are defined in `active_support/core_ext/date_time/calculations.rb`.

The class `DateTime` is a subclass of `Date` so by loading `active_support/core_ext/date/calculations.rb` you inherit these methods and their aliases, except that they will always return datetimes:

    yesterday
    tomorrow
    beginning_of_week (monday, at_beginning_of_week)
    end_on_week (at_end_of_week)
    weeks_ago
    prev_week
    next_week
    months_ago
    months_since
    beginning_of_month (at_beginning_of_month)
    end_of_month (at_end_of_month)
    prev_month
    next_month
    beginning_of_quarter (at_beginning_of_quarter)
    end_of_quarter (at_end_of_quarter)
    beginning_of_year (at_beginning_of_year)
    end_of_year (at_end_of_year)
    years_ago
    years_since
    prev_year
    next_year

The following methods are reimplemented so you do not need to load `active_support/core_ext/date/calculations.rb` for these ones:

    beginning_of_day (midnight, at_midnight, at_beginning_of_day)
    end_of_day
    ago
    since (in)

On the other hand, `advance` and `change` are also defined and support more options, they are documented below.

##### 17.1.1 Named Datetimes

###### 17.1.1.1 `DateTime.current`

Active Support defines `DateTime.current` to be like `Time.now.to_datetime`, except that it honors the user time zone, if defined. It also defines `DateTime.yesterday` and `DateTime.tomorrow`, and the instance predicates `past?`, and `future?` relative to `DateTime.current`.

##### 17.1.2 Other Extensions

###### 17.1.2.1 `seconds_since_midnight`

The method `seconds_since_midnight` returns the number of seconds since midnight:

    now = DateTime.current     # => Mon, 07 Jun 2010 20:26:36 +0000
    now.seconds_since_midnight # => 73596

###### 17.1.2.2 `utc`

The method `utc` gives you the same datetime in the receiver expressed in UTC.

    now = DateTime.current # => Mon, 07 Jun 2010 19:27:52 -0400
    now.utc                # => Mon, 07 Jun 2010 23:27:52 +0000

This method is also aliased as `getutc`.

###### 17.1.2.3 `utc?`

The predicate `utc?` says whether the receiver has UTC as its time zone:

    now = DateTime.now # => Mon, 07 Jun 2010 19:30:47 -0400
    now.utc?           # => false
    now.utc.utc?       # => true

###### 17.1.2.4 `advance`

The most generic way to jump to another datetime is `advance`. This method receives a hash with keys `:years`, `:months`, `:weeks`, `:days`, `:hours`, `:minutes`, and `:seconds`, and returns a datetime advanced as much as the present keys indicate.

    d = DateTime.current
    # => Thu, 05 Aug 2010 11:33:31 +0000
    d.advance(:years => 1, :months => 1, :days => 1, :hours => 1, :minutes => 1, :seconds => 1)
    # => Tue, 06 Sep 2011 12:34:32 +0000

This method first computes the destination date passing `:years`, `:months`, `:weeks`, and `:days` to `Date#advance` documented above. After that, it adjusts the time calling `since` with the number of seconds to advance. This order is relevant, a different ordering would give different datetimes in some edge-cases. The example in `Date#advance` applies, and we can extend it to show order relevance related to the time bits.

If we first move the date bits (that have also a relative order of processing, as documented before), and then the time bits we get for example the following computation:

    d = DateTime.new(2010, 2, 28, 23, 59, 59)
    # => Sun, 28 Feb 2010 23:59:59 +0000
    d.advance(:months => 1, :seconds => 1)
    # => Mon, 29 Mar 2010 00:00:00 +0000

but if we computed them the other way around, the result would be different:

    d.advance(:seconds => 1).advance(:months => 1)
    # => Thu, 01 Apr 2010 00:00:00 +0000

Since `DateTime` is not DST-aware you can end up in a non-existing point in time with no warning or error telling you so.

##### 17.1.3 Changing Components

The method `change` allows you to get a new datetime which is the same as the receiver except for the given options, which may include `:year`, `:month`, `:day`, `:hour`, `:min`, `:sec`, `:offset`, `:start`:

    now = DateTime.current
    # => Tue, 08 Jun 2010 01:56:22 +0000
    now.change(:year => 2011, :offset => Rational(-6, 24))
    # => Wed, 08 Jun 2011 01:56:22 -0600

If hours are zeroed, then minutes and seconds are too (unless they have given values):

    now.change(:hour => 0)
    # => Tue, 08 Jun 2010 00:00:00 +0000

Similarly, if minutes are zeroed, then seconds are too (unless it has given a value):

    now.change(:min => 0)
    # => Tue, 08 Jun 2010 01:00:00 +0000

This method is not tolerant to non-existing dates, if the change is invalid `ArgumentError` is raised:

    DateTime.current.change(:month => 2, :day => 30)
    # => ArgumentError: invalid date

##### 17.1.4 Durations

Durations can be added to and subtracted from datetimes:

    now = DateTime.current
    # => Mon, 09 Aug 2010 23:15:17 +0000
    now + 1.year
    # => Tue, 09 Aug 2011 23:15:17 +0000
    now - 1.week
    # => Mon, 02 Aug 2010 23:15:17 +0000

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

    DateTime.new(1582, 10, 4, 23) + 1.hour
    # => Fri, 15 Oct 1582 00:00:00 +0000

### 18 Extensions to `Time`

#### 18.1 Calculations

All the following methods are defined in `active_support/core_ext/time/calculations.rb`.

Active Support adds to `Time` many of the methods available for `DateTime`:

    past?
    today?
    future?
    yesterday
    tomorrow
    seconds_since_midnight
    change
    advance
    ago
    since (in)
    beginning_of_day (midnight, at_midnight, at_beginning_of_day)
    end_of_day
    beginning_of_week (monday, at_beginning_of_week)
    end_on_week (at_end_of_week)
    weeks_ago
    prev_week
    next_week
    months_ago
    months_since
    beginning_of_month (at_beginning_of_month)
    end_of_month (at_end_of_month)
    prev_month
    next_month
    beginning_of_quarter (at_beginning_of_quarter)
    end_of_quarter (at_end_of_quarter)
    beginning_of_year (at_beginning_of_year)
    end_of_year (at_end_of_year)
    years_ago
    years_since
    prev_year
    next_year

They are analogous. Please refer to their documentation above and take into account the following differences:

- `change` accepts an additional `:usec` option.
- `Time` understands DST, so you get correct DST calculations as in

    Time.zone_default
    # => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>

    # In Barcelona, 2010/03/28 02:00 +0100 becomes 2010/03/28 03:00 +0200 due to DST.
    t = Time.local_time(2010, 3, 28, 1, 59, 59)
    # => Sun Mar 28 01:59:59 +0100 2010
    t.advance(:seconds => 1)
    # => Sun Mar 28 03:00:00 +0200 2010

- If `since` or `ago` jump to a time that can’t be expressed with `Time` a `DateTime` object is returned instead.

##### 18.1.1 `Time.current`

Active Support defines `Time.current` to be today in the current time zone. That’s like `Time.now`, except that it honors the user time zone, if defined. It also defines `Time.yesterday` and `Time.tomorrow`, and the instance predicates `past?`, `today?`, and `future?`, all of them relative to `Time.current`.

When making Time comparisons using methods which honor the user time zone, make sure to use `Time.current` and not `Time.now`. There are cases where the user time zone might be in the future compared to the system time zone, which `Time.today` uses by default. This means `Time.now` may equal `Time.yesterday`.

##### 18.1.2 `all_day`, `all_week`, `all_month`, `all_quarter` and `all_year`

The method `all_day` returns a range representing the whole day of the current time.

    now = Time.current
    # => Mon, 09 Aug 2010 23:20:05 UTC +00:00
    now.all_day
    # => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Mon, 09 Aug 2010 23:59:59 UTC +00:00

Analogously, `all_week`, `all_month`, `all_quarter` and `all_year` all serve the purpose of generating time ranges.

    now = Time.current
    # => Mon, 09 Aug 2010 23:20:05 UTC +00:00
    now.all_week
    # => Mon, 09 Aug 2010 00:00:00 UTC +00:00..Sun, 15 Aug 2010 23:59:59 UTC +00:00
    now.all_month
    # => Sat, 01 Aug 2010 00:00:00 UTC +00:00..Tue, 31 Aug 2010 23:59:59 UTC +00:00
    now.all_quarter
    # => Thu, 01 Jul 2010 00:00:00 UTC +00:00..Thu, 30 Sep 2010 23:59:59 UTC +00:00
    now.all_year
    # => Fri, 01 Jan 2010 00:00:00 UTC +00:00..Fri, 31 Dec 2010 23:59:59 UTC +00:00

#### 18.2 Time Constructors

Active Support defines `Time.current` to be `Time.zone.now` if there’s a user time zone defined, with fallback to `Time.now`:

    Time.zone_default
    # => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>
    Time.current
    # => Fri, 06 Aug 2010 17:11:58 CEST +02:00

Analogously to `DateTime`, the predicates `past?`, and `future?` are relative to `Time.current`.

Use the `local_time` class method to create time objects honoring the user time zone:

    Time.zone_default
    # => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>
    Time.local_time(2010, 8, 15)
    # => Sun Aug 15 00:00:00 +0200 2010

The `utc_time` class method returns a time in UTC:

    Time.zone_default
    # => #<ActiveSupport::TimeZone:0x7f73654d4f38 @utc_offset=nil, @name="Madrid", ...>
    Time.utc_time(2010, 8, 15)
    # => Sun Aug 15 00:00:00 UTC 2010

Both `local_time` and `utc_time` accept up to seven positional arguments: year, month, day, hour, min, sec, usec. Year is mandatory, month and day default to 1, and the rest default to 0.

If the time to be constructed lies beyond the range supported by `Time` in the runtime platform, usecs are discarded and a `DateTime` object is returned instead.

##### 18.2.1 Durations

Durations can be added to and subtracted from time objects:

    now = Time.current
    # => Mon, 09 Aug 2010 23:20:05 UTC +00:00
    now + 1.year
    #  => Tue, 09 Aug 2011 23:21:11 UTC +00:00
    now - 1.week
    # => Mon, 02 Aug 2010 23:21:11 UTC +00:00

They translate to calls to `since` or `advance`. For example here we get the correct jump in the calendar reform:

    Time.utc_time(1582, 10, 3) + 5.days
    # => Mon Oct 18 00:00:00 UTC 1582

### 19 Extensions to `Process`

#### 19.1 `daemon`

Ruby 1.9 provides `Process.daemon`, and Active Support defines it for previous versions. It accepts the same two arguments, whether it should chdir to the root directory (default, true), and whether it should inherit the standard file descriptors from the parent (default, false).

### 20 Extensions to `File`

#### 20.1 `atomic_write`

With the class method `File.atomic_write` you can write to a file in a way that will prevent any reader from seeing half-written content.

The name of the file is passed as an argument, and the method yields a file handle opened for writing. Once the block is done `atomic_write` closes the file handle and completes its job.

For example, Action Pack uses this method to write asset cache files like `all.css`:

    File.atomic_write(joined_asset_path) do |cache|
      cache.write(join_asset_file_contents(asset_paths))
    end

To accomplish this `atomic_write` creates a temporary file. That’s the file the code in the block actually writes to. On completion, the temporary file is renamed, which is an atomic operation on POSIX systems. If the target file exists `atomic_write` overwrites it and keeps owners and permissions.

Note you can’t append with `atomic_write`.

The auxiliary file is written in a standard directory for temporary files, but you can pass a directory of your choice as second argument.

Defined in `active_support/core_ext/file/atomic.rb`.

### 21 Extensions to `Logger`

#### 21.1 `around_[level]`

Takes two arguments, a `before_message` and `after_message` and calls the current level method on the `Logger` instance, passing in the `before_message`, then the specified message, then the `after_message`:

    logger = Logger.new("log/development.log")
    logger.around_info("before", "after") { |logger| logger.info("during") }

#### 21.2 `silence`

Silences every log level lesser to the specified one for the duration of the given block. Log level orders are: debug, info, error and fatal.

    logger = Logger.new("log/development.log")
    logger.silence(Logger::INFO) do
      logger.debug("In space, no one can hear you scream.")
      logger.info("Scream all you want, small mailman!")
    end

#### 21.3 `datetime_format=`

Modifies the datetime format output by the formatter class associated with this logger. If the formatter class does not have a `datetime_format` method then this is ignored.

    class Logger::FormatWithTime < Logger::Formatter
      cattr_accessor(:datetime_format) { "%Y%m%d%H%m%S" }

      def self.call(severity, timestamp, progname, msg)
        "#{timestamp.strftime(datetime_format)} -- #{String === msg ? msg : msg.inspect}\n"
      end
    end

    logger = Logger.new("log/development.log")
    logger.formatter = Logger::FormatWithTime
    logger.info("<- is the current time")

Defined in `active_support/core_ext/logger.rb`.

### 22 Extensions to `NameError`

Active Support adds `missing_name?` to `NameError`, which tests whether the exception was raised because of the name passed as argument.

The name may be given as a symbol or string. A symbol is tested against the bare constant name, a string is against the fully-qualified constant name.

A symbol can represent a fully-qualified constant name as in `:"ActiveRecord::Base"`, so the behavior for symbols is defined for convenience, not because it has to be that way technically.

For example, when an action of `PostsController` is called Rails tries optimistically to use `PostsHelper`. It is OK that the helper module does not exist, so if an exception for that constant name is raised it should be silenced. But it could be the case that `posts_helper.rb` raises a `NameError` due to an actual unknown constant. That should be reraised. The method `missing_name?` provides a way to distinguish both cases:

    def default_helper_module!
      module_name = name.sub(/Controller$/, '')
      module_path = module_name.underscore
      helper module_path
    rescue MissingSourceFile => e
      raise e unless e.is_missing? "#{module_path}_helper"
    rescue NameError => e
      raise e unless e.missing_name? "#{module_name}Helper"
    end

Defined in `active_support/core_ext/name_error.rb`.

### 23 Extensions to `LoadError`

Active Support adds `is_missing?` to `LoadError`, and also assigns that class to the constant `MissingSourceFile` for backwards compatibility.

Given a path name `is_missing?` tests whether the exception was raised due to that particular file (except perhaps for the “.rb” extension).

For example, when an action of `PostsController` is called Rails tries to load `posts_helper.rb`, but that file may not exist. That’s fine, the helper module is not mandatory so Rails silences a load error. But it could be the case that the helper module does exist and in turn requires another library that is missing. In that case Rails must reraise the exception. The method `is_missing?` provides a way to distinguish both cases:

    def default_helper_module!
      module_name = name.sub(/Controller$/, '')
      module_path = module_name.underscore
      helper module_path
    rescue MissingSourceFile => e
      raise e unless e.is_missing? "helpers/#{module_path}_helper"
    rescue NameError => e
      raise e unless e.missing_name? "#{module_name}Helper"
    end

Defined in `active_support/core_ext/load_error.rb`.

### Feedback

You're encouraged to help improve the quality of this guide.

If you see any typos or factual errors you are confident to patch, please clone docrails and push the change yourself. That branch of Rails has public write access. Commits are still reviewed, but that happens after you've submitted your contribution. docrails is cross-merged with master periodically.

You may also find incomplete content, or stuff that is not up to date. Please do add any missing documentation for master. Check the Ruby on Rails Guides Guidelines for style and conventions.

If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue.

And last but not least, any kind of discussion regarding Ruby on Rails documentation is very welcome in the rubyonrails-docs mailing list.