.. _changeset-system:
.. highlight:: python

**************************
Database schema migrations
**************************

.. currentmodule:: migrate.changeset.schema

Importing :mod:`migrate.changeset` adds some new methods to existing
SQLAlchemy objects, as well as creating functions of its own. Most operations
can be done either by a method or a function. Methods match
SQLAlchemy's existing API and are more intuitive when the object is
available; functions allow one to make changes when only the name of
an object is available (for example, adding a column to a table in the
database without having to load that table into Python).

Changeset operations can be used independently of SQLAlchemy Migrate's
:ref:`versioning <versioning-system>`.

For more information, see the API documentation for
:mod:`migrate.changeset`.

.. _summary-changeset-api:

Here are some direct links to the relevent sections of the API documentations:


* :meth:`Create a column <ChangesetColumn.create>`
* :meth:`Drop a column <ChangesetColumn.drop>`
* :meth:`Alter a column <ChangesetColumn.alter>` (follow a link for list of supported changes)
* :meth:`Rename a table <ChangesetTable.rename>`
* :meth:`Rename an index <ChangesetIndex.rename>`
* :meth:`Create primary key constraint <migrate.changeset.constraint.PrimaryKeyConstraint>`
* :meth:`Drop primary key constraint <migrate.changeset.constraint.PrimaryKeyConstraint.drop>`
* :meth:`Create foreign key contraint <migrate.changeset.constraint.ForeignKeyConstraint.create>`
* :meth:`Drop foreign key constraint <migrate.changeset.constraint.ForeignKeyConstraint.drop>`
* :meth:`Create unique key contraint <migrate.changeset.constraint.UniqueConstraint.create>`
* :meth:`Drop unique key constraint <migrate.changeset.constraint.UniqueConstraint.drop>`
* :meth:`Create check key contraint <migrate.changeset.constraint.CheckConstraint.create>`
* :meth:`Drop check key constraint <migrate.changeset.constraint.CheckConstraint.drop>`


.. note::

  Many of the schema modification methods above take an
  ``alter_metadata`` keyword parameter. This parameter defaults to
  ``True``.

The following sections give examples of how to make various kinds of
schema changes.

Column
======

Given a standard SQLAlchemy table::

 table = Table('mytable', meta,
    Column('id', Integer, primary_key=True),
 )
 table.create()

.. _column-create:

You can create a column with :meth:`~ChangesetColumn.create`::

 col = Column('col1', String, default='foobar')
 col.create(table, populate_default=True)

 # Column is added to table based on its name
 assert col is table.c.col1

 # col1 is populated with 'foobar' because of `populate_default`

.. _column-drop:

.. note::

  You can pass `primary_key_name`, `index_name`
  and `unique_name` to the :meth:`~ChangesetColumn.create` method to issue ``ALTER TABLE ADD
  CONSTRAINT`` after changing the column. 

  For multi columns constraints and other advanced configuration,
  check the :ref:`constraint tutorial <constraint-tutorial>`. 

  .. versionadded:: 0.6.0

You can drop a column with :meth:`~ChangesetColumn.drop`::

 col.drop()


.. _column-alter:

You can alter a column with :meth:`~ChangesetColumn.alter`::

 col.alter(name='col2')

 # Renaming a column affects how it's accessed by the table object
 assert col is table.c.col2

 # Other properties can be modified as well
 col.alter(type=String(42), default="life, the universe, and everything", nullable=False)

 # Given another column object, col1.alter(col2), col1 will be changed to match col2
 col.alter(Column('col3', String(77), nullable=True))
 assert col.nullable
 assert table.c.col3 is col

.. deprecated:: 0.6.0
    Passing a :class:`~sqlalchemy.schema.Column` to
    :meth:`ChangesetColumn.alter` is deprecated. Pass in explicit
    parameters, such as `name` for a new column name and `type` for a
    new column type, instead. Do **not** include any parameters that
    are not changed.

.. _table-rename:

Table
=====

SQLAlchemy includes support for `creating and dropping`__ tables..

Tables can be renamed with :meth:`~ChangesetTable.rename`::

 table.rename('newtablename')

.. __: http://www.sqlalchemy.org/docs/05/metadata.html#creating-and-dropping-database-tables
.. currentmodule:: migrate.changeset.constraint


.. _index-rename:

Index
=====

SQLAlchemy supports `creating and dropping`__ indexes.

Indexes can be renamed using :meth:`~migrate.changeset.schema.ChangesetIndex.rename`::

 index.rename('newindexname')

.. __: http://www.sqlalchemy.org/docs/05/metadata.html#indexes


.. _constraint-tutorial:

Constraint
==========

.. currentmodule:: migrate.changeset.constraint

SQLAlchemy supports creating or dropping constraints at the same time
a table is created or dropped. SQLAlchemy Migrate adds support for
creating and dropping
:class:`~sqlalchemy.schema.PrimaryKeyConstraint`,
:class:`~sqlalchemy.schema.ForeignKeyConstraint`,
:class:`~sqlalchemy.schema.CheckConstraint` and
:class:`~sqlalchemy.schema.UniqueConstraint` constraints independently
using ``ALTER TABLE`` statements.

The following rundowns are true for all constraints classes:

1. Make sure you import the relevent constrain class Migrate and not
   from SQLAlchemy, for example::

    from migrate.changeset.constraint import ForeignKeyConstraint

   The classes in that module have the extra
   :meth:`~ConstraintChangeset.create` and
   :meth:`~ConstraintChangeset.drop` methods.

2. You can also use Constraints as in SQLAlchemy. In this case passing table argument explicitly is required::

	cons = PrimaryKeyConstraint('id', 'num', table=self.table)

	# Create the constraint
	cons.create()

	# Drop the constraint
	cons.drop()

   You can also pass in :class:`~sqlalchemy.schema.Column` objects (and table argument can be left out)::

	cons = PrimaryKeyConstraint(col1, col2)

3. Some dialects support ``CASCADE`` option when dropping constraints::

	cons = PrimaryKeyConstraint(col1, col2)

	# Create the constraint
	cons.create()

	# Drop the constraint
	cons.drop(cascade=True)


.. note::
	 SQLAlchemy Migrate will try to guess the name of the
	 constraints for databases, but if it's something other than
	 the default, you'll need to give its name. Best practice is
	 to always name your constraints. Note that Oracle requires
	 that you state the name of the constraint to be created or dropped.


Examples
---------

Primary key constraints::

 from migrate.changeset.constraint import PrimaryKeyConstraint

 cons = PrimaryKeyConstraint(col1, col2)

 # Create the constraint
 cons.create()

 # Drop the constraint
 cons.drop()

Foreign key constraints::

 from migrate.changeset.constraint import ForeignKeyConstraint

 cons = ForeignKeyConstraint([table.c.fkey], [othertable.c.id])

 # Create the constraint
 cons.create()

 # Drop the constraint
 cons.drop()

Check constraints::

	from migrate.changeset.constraint import CheckConstraint

	cons = CheckConstraint('id > 3', columns=[table.c.id])

	# Create the constraint
	cons.create()

	# Drop the constraint
	cons.drop()

Unique constraints::

	from migrate.changeset.constraint import UniqueConstraint

	cons = UniqueConstraint('id', 'age', table=self.table)

	# Create the constraint
	cons.create()

	# Drop the constraint
	cons.drop()
