.. _aiopg-rpc:

:mod:`aiopg.sa` --- support for SQLAlchemy functional SQL layer
===============================================================

.. module:: aiopg.sa
   :synopsis: support for SQLAlchemy functional SQL layer
.. currentmodule:: aiopg.sa


Intro
-----

While :ref:`core API <aiopg-core>` provides a core support for access
to :term:`PostgreSQL` database, I found manipulations with raw SQL
strings too annoying.

Fortunately we can use excellent :ref:`core_toplevel` as **SQL query builder**.

Example::

    import asyncio
    from aiopg.sa import create_engine
    import sqlalchemy as sa


    metadata = sa.MetaData()

    tbl = sa.Table('tbl', metadata,
                   sa.Column('id', sa.Integer, primary_key=True),
                   sa.Column('val', sa.String(255)))


    async def create_table(engine):
        async with engine.acquire() as conn:
            await conn.execute('DROP TABLE IF EXISTS tbl')
            await conn.execute('''CREATE TABLE tbl (
                                      id serial PRIMARY KEY,
                                      val varchar(255))''')


    async def go():
        async with create_engine(user='aiopg',
                                 database='aiopg',
                                 host='127.0.0.1',
                                 password='passwd') as engine:

            await create_table(engine)
            async with engine.acquire() as conn:
                await conn.execute(tbl.insert().values(val='abc'))

                async for row in conn.execute(tbl.select()):
                    print(row.id, row.val)


    loop = asyncio.get_event_loop()
    loop.run_until_complete(go())

So you can execute SQL query built by
``tbl.insert().values(val='abc')`` or ``tbl.select()`` expressions.

:term:`sqlalchemy` has rich and very powerful set of SQL construction
functions, please read :ref:`tutorial <core_toplevel>` for full list
of available operations.

Also we provide SQL transactions support. Please take a look on
:meth:`SAConnection.begin` method and family.

Engine
------

.. cofunction:: create_engine(dsn=None, *, minsize=1, maxsize=10, loop=None, \
                              dialect=dialect, timeout=60, **kwargs)
   :coroutine:
   :async-with:

   Crate an :class:`Engine` instance with embedded connection pool.

   The pool has *minsize* opened connections to :term:`PostgreSQL` server.


.. data:: dialect

   An instance of :term:`SQLAlchemy` dialect set up for :term:`psycopg2` usage.

   An :class:`sqlalchemy.engine.interfaces.Dialect` instance.

   .. seealso:: :mod:`sqlalchemy.dialects.postgresql.psycopg2`
                psycopg2 dialect.


.. class:: Engine

   Connects a :class:`aiopg.Pool` and
   :class:`sqlalchemy.engine.interfaces.Dialect` together to provide a
   source of database connectivity and behavior.

   An :class:`Engine` object is instantiated publicly using the
   :func:`create_engine` coroutine.


   .. attribute:: dialect

      A :class:`sqlalchemy.engine.interfaces.Dialect` for the engine,
      readonly property.

   .. attribute:: name

      A name of the dialect, readonly property.

   .. attribute:: driver

      A driver of the dialect, readonly property.

   .. attribute:: dsn

      DSN connection info, readonly property.

      .. seealso::

         `psycopg2 connection.dsn
         <http://initd.org/psycopg/docs/connection.html#connection.dsn>`_
         attribute.

   .. attribute:: minsize

      A minimal size of the pool (*read-only*), ``1`` by default.

   .. attribute:: maxsize

      A maximal size of the pool (*read-only*), ``10`` by default.

   .. attribute:: size

      A current size of the pool (*readonly*). Includes used and free
      connections.

   .. attribute:: freesize

      A count of free connections in the pool (*readonly*).

   .. attribute:: timeout

      A read-only float representing default timeout for operations
      for connections from pool.

   .. method:: close()

      Close engine.

      Mark all engine connections to be closed on getting back to engine.
      Closed engine doesn't allow to acquire new connections.

      If you want to wait for actual closing of acquired connection please
      call :meth:`wait_closed` after :meth:`close`.

      .. warning:: The method is not a :ref:`coroutine <coroutine>`.

   .. method:: terminate()

      Terminate engine.

      Close engine's pool with instantly closing all acquired connections also.

      :meth:`wait_closed` should be called after :meth:`terminate` for
      waiting for actual finishing.

      .. warning:: The method is not a :ref:`coroutine <coroutine>`.

   .. comethod:: wait_closed()

      A :ref:`coroutine <coroutine>` that waits for releasing and
      closing all acquired connections.

      Should be called after :meth:`close` for waiting for actual engine
      closing.

   .. comethod:: acquire()
      :coroutine:
      :async-with:

      Get a connection from pool.

      This method is a :ref:`coroutine <coroutine>`.

      Returns a :class:`SAConnection` instance. Result of this method could
      be used as async contex manager::

            async with engine.acquire() as conn:
                await conn.execute(tbl.insert().values(val='abc'))

      .. warning:: nested ``acquire()`` might lead to deadlocks.

   .. method:: release()

      Revert back connection *conn* to pool.

      .. warning:: The method is not a :ref:`coroutine <coroutine>`.



Connection
----------

.. class:: SAConnection

   A wrapper for :class:`aiopg.Connection` instance.

   The class provides methods for executing *SQL queries* and working with
   *SQL transactions*.

   .. comethod:: execute(query, *multiparams, **params)
      :coroutine:
      :async-for:

      Executes a *SQL* *query* with optional parameters.

      :param query: a SQL query string or any :term:`sqlalchemy`
                    expression (see :ref:`core_toplevel`)

      :param \*multiparams/\**params: represent bound parameter values
       to be used in the execution.  Typically, the format is either a
       dictionary passed to \*multiparams::

           await conn.execute(
               table.insert(),
               {"id":1, "value":"v1"}
           )

       ...or individual key/values interpreted by \**params::

           await conn.execute(
               table.insert(), id=1, value="v1"
           )

       In the case that a plain SQL string is passed, a tuple or
       individual values in \*multiparams may be passed::

           await conn.execute(
               "INSERT INTO table (id, value) VALUES (%d, %s)",
               (1, "v1")
           )

           await conn.execute(
               "INSERT INTO table (id, value) VALUES (%s, %s)",
               1, "v1"
           )

      Result value for ``SELECT`` statements may be iterated immediately::

           async for row conn.execute(tbl.select()):
               print(row.id, row.name, row.surname)

      :returns: :class:`ResultProxy` instance with results of SQL
                query execution.

   .. comethod:: scalar(query, *multiparams, **params)

      Executes a *SQL* *query* and returns a scalar value.

      .. seealso:: :meth:`SAConnection.execute` and :meth:`ResultProxy.scalar`.

   .. attribute:: closed

      The readonly property that returns ``True`` if connections is closed.

   .. comethod:: begin()
      :coroutine:
      :async-with:

      Begin a transaction and return a transaction handle.

      This method is a :ref:`coroutine <coroutine>`.

      The returned object is an instance of :class:`.Transaction`.
      This object represents the "scope" of the transaction,
      which completes when either the :meth:`.Transaction.rollback`
      or :meth:`.Transaction.commit` method is called.

      Nested calls to :meth:`.begin` on the same :class:`.SAConnection`
      will return new :class:`.Transaction` objects that represent
      an emulated transaction within the scope of the enclosing
      transaction, that is::

          trans = await conn.begin()   # outermost transaction
          trans2 = await conn.begin()  # "inner"
          await trans2.commit()        # does nothing
          await trans.commit()         # actually commits

      Calls to :meth:`.Transaction.commit` only have an effect
      when invoked via the outermost :class:`.Transaction` object, though the
      :meth:`.Transaction.rollback` method of any of the
      :class:`.Transaction` objects will roll back the
      transaction.

      .. seealso::

         :meth:`.SAConnection.begin_nested` - use a SAVEPOINT

         :meth:`.SAConnection.begin_twophase` - use a two phase (XA)
                 transaction

   .. comethod:: begin_nested()
      :coroutine:
      :async-with:

      Begin a nested transaction and return a transaction handle.

      The returned object is an instance of :class:`.NestedTransaction`.

      Any transaction in the hierarchy may ``commit`` and
      ``rollback``, however the outermost transaction still controls
      the overall ``commit`` or ``rollback`` of the transaction of a
      whole. It utilizes SAVEPOINT facility of :term:`PostgreSQL` server.

      .. seealso::

         :meth:`.SAConnection.begin`, :meth:`.SAConnection.begin_twophase`.

   .. comethod:: begin_twophase(xid=None)
      :coroutine:
      :async-with:

      Begin a two-phase or XA transaction and return a transaction
      handle.

      The returned object is an instance of
      :class:`.TwoPhaseTransaction`, which in addition to the methods
      provided by :class:`.Transaction`, also provides a
      :meth:`~.TwoPhaseTransaction.prepare` method.

      :param xid: the two phase transaction id.  If not supplied, a
          random id will be generated.

      .. seealso::
         :meth:`.SAConnection.begin`, :meth:`.SAConnection.begin_twophase`.

   .. comethod:: recover_twophase()

      Return a list of prepared twophase transaction ids.

   .. comethod:: rollback_prepared(xid)

      Rollback prepared twophase transaction *xid*.

   .. comethod:: commit_prepared(xid)

      Commit prepared twophase transaction *xid*.

   .. attribute:: in_transaction

      The readonly property that returns ``True`` if a transaction is
      in progress.

   .. comethod:: close()

      Close this :class:`SAConnection`.

      This results in a release of the underlying database
      resources, that is, the :class:`aiopg.Connection` referenced
      internally. The :class:`aiopg.Connection` is typically restored
      back to the connection-holding :class:`aiopg.Pool` referenced
      by the :class:`.Engine` that produced this
      :class:`SAConnection`. Any transactional state present on
      the :class:`aiopg.Connection` is also unconditionally released via
      calling :meth:`Transaction.rollback` method.

      After :meth:`~.SAConnection.close` is called, the
      :class:`.SAConnection` is permanently in a closed state,
      and will allow no further operations.


ResultProxy
-----------

.. class:: ResultProxy

   Wraps a *DB-API like* :class:`Cursor` object to provide easier
   access to row columns.

   Individual columns may be accessed by their integer position,
   case-sensitive column name, or by :class:`sqlalchemy.schema.Column``
   object. e.g.::

      async for row in conn.execute(...):
          col1 = row[0]    # access via integer position
          col2 = row['col2']   # access via name
          col3 = row[mytable.c.mycol] # access via Column object.

   :class:`ResultProxy` also handles post-processing of result column
   data using :class:`sqlalchemy.types.TypeEngine` objects, which are
   referenced from the originating SQL statement that produced this
   result set.

   .. attribute:: dialect

      The readonly property that returns
      :class:`sqlalchemy.engine.interfaces.Dialect` dialect
      for the :class:`ResultProxy` instance.

      .. seealso:: :data:`dialect` global data.

   .. method:: keys()

      Return the current set of string keys for rows.

   .. attribute:: rowcount

      The readonly property that returns the 'rowcount' for this result.

      The 'rowcount' reports the number of rows *matched*
      by the WHERE criterion of an UPDATE or DELETE statement.

      .. note::

         Notes regarding :attr:`ResultProxy.rowcount`:


         * This attribute returns the number of rows *matched*,
           which is not necessarily the same as the number of rows
           that were actually *modified* - an UPDATE statement, for example,
           may have no net change on a given row if the SET values
           given are the same as those present in the row already.
           Such a row would be matched but not modified.

         * :attr:`ResultProxy.rowcount` is *only* useful in conjunction
           with an UPDATE or DELETE statement.  Contrary to what the Python
           DBAPI says, it does *not* return the
           number of rows available from the results of a SELECT statement
           as DBAPIs cannot support this functionality when rows are
           unbuffered.

         * Statements that use RETURNING does not return a correct
           rowcount.

   .. attribute:: returns_rows

        A readonly property that returns ``True`` if this
        :class:`ResultProxy` returns rows.

        I.e. if it is legal to call the methods
        :meth:`ResultProxy.fetchone`,
        :meth:`ResultProxy.fetchmany`,
        :meth:`ResultProxy.fetchall`.

   .. attribute:: closed

        Return ``True`` if this :class:`ResultProxy` is closed (no
        pending rows in underlying cursor).

   .. method:: close()

      Close this :class:`ResultProxy`.

      Closes the underlying :class:`aiopg.Cursor` corresponding to the
      execution.

      Note that any data cached within this :class:`ResultProxy` is
      still available.  For some types of results, this may include
      buffered rows.

      This method is called automatically when:

      * all result rows are exhausted using the fetchXXX() methods.
      * cursor.description is None.

   .. comethod:: fetchall()

      Fetch all rows, just like :meth:`aiopg.Cursor.fetchall`.

      The connection is closed after the call.

      Returns a list of :class:`RowProxy`.

   .. comethod:: fetchone()

      Fetch one row, just like :meth:`aiopg.Cursor.fetchone`.

      If a row is present, the cursor remains open after this is called.

      Else the cursor is automatically closed and ``None`` is returned.

      Returns an :class:`RowProxy` instance or ``None``.

   .. comethod:: fetchmany(size=None)

      Fetch many rows, just like :meth:`aiopg.Cursor.fetchmany`.

      If rows are present, the cursor remains open after this is called.

      Else the cursor is automatically closed and an empty list is returned.

      Returns a list of :class:`RowProxy`.

   .. comethod:: first()

      Fetch the first row and then close the result set unconditionally.

      Returns ``None`` if no row is present or an :class:`RowProxy` instance.

   .. comethod:: scalar()

      Fetch the first column of the first row, and close the result set.

      Returns ``None`` if no row is present or an :class:`RowProxy` instance.


.. class:: RowProxy

   A :class:`collections.abc.Mapping` for representing a row in query result.

   Keys are column names, values are result values.

   Individual columns may be accessed by their integer position,
   case-sensitive column name, or by :class:`sqlalchemy.schema.Column``
   object.

   Has overloaded operators ``__eq__`` and ``__ne__`` for comparing two rows.

   The :class:`RowProxy` is *not hashable*.

   ..method:: as_tuple()

   Return a tuple with values from :meth:`RowProxy.values`.


Transaction objects
-------------------

.. class:: Transaction

   Represent a database transaction in progress.

   The :class:`Transaction` object is procured by
   calling the :meth:`SAConnection.begin` method of
   :class:`SAConnection`::

       async with engine.acquire() as conn:
           async with conn.begin() as tr:
               await conn.execute("insert into x (a, b) values (1, 2)")

   The object provides :meth:`.rollback` and :meth:`.commit`
   methods in order to control transaction boundaries. Contex manager will
   invoke :meth:`.rollback` in case of exception in contex managers code block
   and :meth:`.commit` - in case of success.

   .. seealso::

      :meth:`SAConnection.begin`, :meth:`SAConnection.begin_twophase`,
      :meth:`SAConnection.begin_nested`.

   .. attribute:: is_active

      A readonly property that returns ``True`` if a transaction is active.

   .. attribute:: connection

      A readonly property that returns :class:`SAConnection` for transaction.

   .. comethod:: close()

      Close this :class:`Transaction`.

      If this transaction is the base transaction in a begin/commit
      nesting, the transaction will :meth:`Transaction.rollback`.
      Otherwise, the method returns.

      This is used to cancel a :class:`Transaction` without affecting
      the scope of an enclosing transaction.

   .. comethod:: rollback()

      Roll back this :class:`Transaction`.

   .. comethod:: commit()

      Commit this :class:`Transaction`.


.. class:: NestedTransaction

   Represent a 'nested', or SAVEPOINT transaction.

   A new :class:`NestedTransaction` object may be procured
   using the :meth:`SAConnection.begin_nested` method.

   The interface is the same as that of :class:`Transaction`.

   .. seealso:: PostgreSQL commands for nested transactions:

      * SAVEPOINT_
      * `RELEASE SAVEPOINT`_
      * `ROLLBACK TO SAVEPOINT`_

.. _SAVEPOINT: http://www.postgresql.org/docs/current/static/sql-savepoint.html

.. _RELEASE SAVEPOINT:
   http://www.postgresql.org/docs/current/static/sql-release-savepoint.html

.. _ROLLBACK TO SAVEPOINT:
   http://www.postgresql.org/docs/current/static/sql-rollback-to.html


.. class:: TwoPhaseTransaction

   Represent a two-phase transaction.

   A new :class:`TwoPhaseTransaction` object may be procured
   using the :meth:`SAConnection.begin_twophase` method.

   The interface is the same as that of :class:`Transaction`
   with the addition of the :meth:`TwoPhaseTransaction.prepare` method.

   .. attribute:: xid

      A readonly property that returns twophase transaction id.

   .. comethod:: prepare()

      Prepare this :class:`TwoPhaseTransaction`.

      After a PREPARE, the transaction can be committed.

   .. seealso:: PostgreSQL commands for two phase transactions:

      * `PREPARE TRANSACTION`_
      * `COMMIT PREPARED`_
      * `ROLLBACK PREPARED`_

.. _PREPARE TRANSACTION:
   http://www.postgresql.org/docs/current/static/sql-prepare-transaction.html

.. _COMMIT PREPARED:
   http://www.postgresql.org/docs/current/static/sql-commit-prepared.html

.. _ROLLBACK PREPARED:
   http://www.postgresql.org/docs/current/static/sql-rollback-prepared.html
