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## Rails nested model forms

Creating a form for a model and its associations can become quite tedious. Therefore Rails provides helpers to assist in dealing with the complexities of generating these forms and the required CRUD operations to create, update, and destroy associations.

In this guide you will:

- do stuff

### Chapters

1. Model setup
   - ActiveRecord::Base model
   - Custom model
2. Views
   - Controller code
   - Form code

This guide assumes the user knows how to use the Rails form helpers in general. Also, it’s not an API reference. For a complete reference please visit the Rails API documentation.

### 1 Model setup

To be able to use the nested model functionality in your forms, the model will need to support some basic operations.

First of all, it needs to define a writer method for the attribute that corresponds to the association you are building a nested model form for. The `fields_for` form helper will look for this method to decide whether or not a nested model form should be build.

If the associated object is an array a form builder will be yielded for each object, else only a single form builder will be yielded.

Consider a Person model with an associated Address. When asked to yield a nested FormBuilder for the `:address` attribute, the `fields_for` form helper will look for a method on the Person instance named `address_attributes=`.

#### 1.1 ActiveRecord::Base model

For an ActiveRecord::Base model and association this writer method is commonly defined with the `accepts_nested_attributes_for` class method:

##### 1.1.1 has_one

    class Person < ActiveRecord::Base
      has_one :address
      accepts_nested_attributes_for :address
    end

##### 1.1.2 belongs_to

    class Person < ActiveRecord::Base
      belongs_to :firm
      accepts_nested_attributes_for :firm
    end

##### 1.1.3 has_many / has_and_belongs_to_many

    class Person < ActiveRecord::Base
      has_many :projects
      accepts_nested_attributes_for :projects
    end

#### 1.2 Custom model

As you might have inflected from this explanation, you don’t necessarily need an ActiveRecord::Base model to use this functionality. The following examples are sufficient to enable the nested model form behaviour:

##### 1.2.1 Single associated object

    class Person
      def address
        Address.new
      end

      def address_attributes=(attributes)
        # ...
      end
    end

##### 1.2.2 Association collection

    class Person
      def projects
        [Project.new, Project.new]
      end

      def projects_attributes=(attributes)
        # ...
      end
    end

See (TODO) in the advanced section for more information on how to deal with the CRUD operations in your custom model.

### 2 Views

#### 2.1 Controller code

A nested model form will only be built if the associated object(s) exist. This means that for a new model instance you would probably want to build the associated object(s) first.

Consider the following typical RESTful controller which will prepare a new Person instance and its `address` and `projects` associations before rendering the `new` template:

    class PeopleController < ActionController:Base
      def new
        @person = Person.new
        @person.built_address
        2.times { @person.projects.build }
      end

      def create
        @person = Person.new(params[:person])
        if @person.save
          # ...
        end
      end
    end

Obviously the instantiation of the associated object(s) can become tedious and not DRY, so you might want to move that into the model itself. ActiveRecord::Base provides an `after_initialize` callback which is a good way to refactor this.

#### 2.2 Form code

Now that you have a model instance, with the appropriate methods and associated object(s), you can start building the nested model form.

##### 2.2.1 Standard form

Start out with a regular RESTful form:

    <%= form_for @person do |f| %>
      <%= f.text_field :name %>
    <% end %>

This will generate the following html:

    <form action="/people" class="new_person" id="new_person" method="post">
      <input id="person_name" name="person[name]" size="30" type="text" />
    </form>

##### 2.2.2 Nested form for a single associated object

Now add a nested form for the `address` association:

    <%= form_for @person do |f| %>
      <%= f.text_field :name %>

      <%= f.fields_for :address do |af| %>
        <%= af.text_field :street %>
      <% end %>
    <% end %>

This generates:

    <form action="/people" class="new_person" id="new_person" method="post">
      <input id="person_name" name="person[name]" size="30" type="text" />

      <input id="person_address_attributes_street" name="person[address_attributes][street]" size="30" type="text" />
    </form>

Notice that `fields_for` recognized the `address` as an association for which a nested model form should be built by the way it has namespaced the `name` attribute.

When this form is posted the Rails parameter parser will construct a hash like the following:

    {
      "person" => {
        "name" => "Eloy Duran",
        "address_attributes" => {
          "street" => "Nieuwe Prinsengracht"
        }
      }
    }

That’s it. The controller will simply pass this hash on to the model from the `create` action. The model will then handle building the `address` association for you and automatically save it when the parent (`person`) is saved.

##### 2.2.3 Nested form for a collection of associated objects

The form code for an association collection is pretty similar to that of a single associated object:

    <%= form_for @person do |f| %>
      <%= f.text_field :name %>

      <%= f.fields_for :projects do |pf| %>
        <%= pf.text_field :name %>
      <% end %>
    <% end %>

Which generates:

    <form action="/people" class="new_person" id="new_person" method="post">
      <input id="person_name" name="person[name]" size="30" type="text" />

      <input id="person_projects_attributes_0_name" name="person[projects_attributes][0][name]" size="30" type="text" />
      <input id="person_projects_attributes_1_name" name="person[projects_attributes][1][name]" size="30" type="text" />
    </form>

As you can see it has generated 2 `project name` inputs, one for each new `project` that was built in the controller’s `new` action. Only this time the `name` attribute of the input contains a digit as an extra namespace. This will be parsed by the Rails parameter parser as:

    {
      "person" => {
        "name" => "Eloy Duran",
        "projects_attributes" => {
          "0" => { "name" => "Project 1" },
          "1" => { "name" => "Project 2" }
        }
      }
    }

You can basically see the `projects_attributes` hash as an array of attribute hashes, one for each model instance.

The reason that `fields_for` constructed a form which would result in a hash instead of an array is that it won’t work for any forms nested deeper than one level deep.

You can however pass an array to the writer method generated by `accepts_nested_attributes_for` if you’re using plain Ruby or some other API access. See (TODO) for more info and example.

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