====================
Migration Operations
====================

.. module:: django.db.migrations.operations

Migration files are composed of one or more ``Operation``\s, objects that
declaratively record what the migration should do to your database.

Django also uses these ``Operation`` objects to work out what your models
looked like historically, and to calculate what changes you've made to
your models since the last migration so it can automatically write
your migrations; that's why they're declarative, as it means Django can
easily load them all into memory and run through them without touching
the database to work out what your project should look like.

There are also more specialized ``Operation`` objects which are for things like
:ref:`data migrations <data-migrations>` and for advanced manual database
manipulation. You can also write your own ``Operation`` classes if you want
to encapsulate a custom change you commonly make.

If you need an empty migration file to write your own ``Operation`` objects
into, use ``python manage.py makemigrations --empty yourappname``, but be aware
that manually adding schema-altering operations can confuse the migration
autodetector and make resulting runs of :djadmin:`makemigrations` output
incorrect code.

All of the core Django operations are available from the
``django.db.migrations.operations`` module.

For introductory material, see the :doc:`migrations topic guide
</topics/migrations>`.

Schema Operations
=================

``CreateModel``
---------------

.. class:: CreateModel(name, fields, options=None, bases=None, managers=None)

Creates a new model in the project history and a corresponding table in the
database to match it.

``name`` is the model name, as would be written in the ``models.py`` file.

``fields`` is a list of 2-tuples of ``(field_name, field_instance)``.
The field instance should be an unbound field (so just
``models.CharField(...)``, rather than a field taken from another model).

``options`` is an optional dictionary of values from the model's ``Meta`` class.

``bases`` is an optional list of other classes to have this model inherit from;
it can contain both class objects as well as strings in the format
``"appname.ModelName"`` if you want to depend on another model (so you inherit
from the historical version). If it's not supplied, it defaults to inheriting
from the standard ``models.Model``.

``managers`` takes a list of 2-tuples of ``(manager_name, manager_instance)``.
The first manager in the list will be the default manager for this model during
migrations.

``DeleteModel``
---------------

.. class:: DeleteModel(name)

Deletes the model from the project history and its table from the database.

``RenameModel``
---------------

.. class:: RenameModel(old_name, new_name)

Renames the model from an old name to a new one.

You may have to manually add
this if you change the model's name and quite a few of its fields at once; to
the autodetector, this will look like you deleted a model with the old name
and added a new one with a different name, and the migration it creates will
lose any data in the old table.

``AlterModelTable``
-------------------

.. class:: AlterModelTable(name, table)

Changes the model's table name (the :attr:`~django.db.models.Options.db_table`
option on the ``Meta`` subclass).

``AlterUniqueTogether``
-----------------------

.. class:: AlterUniqueTogether(name, unique_together)

Changes the model's set of unique constraints (the
:attr:`~django.db.models.Options.unique_together` option on the ``Meta``
subclass).

``AlterIndexTogether``
----------------------

.. class:: AlterIndexTogether(name, index_together)

Changes the model's set of custom indexes (the
:attr:`~django.db.models.Options.index_together` option on the ``Meta``
subclass).

``AlterOrderWithRespectTo``
---------------------------

.. class:: AlterOrderWithRespectTo(name, order_with_respect_to)

Makes or deletes the ``_order`` column needed for the
:attr:`~django.db.models.Options.order_with_respect_to` option on the ``Meta``
subclass.

``AlterModelOptions``
---------------------

.. class:: AlterModelOptions(name, options)

Stores changes to miscellaneous model options (settings on a model's ``Meta``)
like ``permissions`` and ``verbose_name``. Does not affect the database, but
persists these changes for :class:`RunPython` instances to use. ``options``
should be a dictionary mapping option names to values.

``AlterModelManagers``
----------------------

.. class:: AlterModelManagers(name, managers)

Alters the managers that are available during migrations.

``AddField``
------------

.. class:: AddField(model_name, name, field, preserve_default=True)

Adds a field to a model. ``model_name`` is the model's name, ``name`` is
the field's name, and ``field`` is an unbound Field instance (the thing
you would put in the field declaration in ``models.py`` - for example,
``models.IntegerField(null=True)``.

The ``preserve_default`` argument indicates whether the field's default
value is permanent and should be baked into the project state (``True``),
or if it is temporary and just for this migration (``False``) - usually
because the migration is adding a non-nullable field to a table and needs
a default value to put into existing rows. It does not affect the behavior
of setting defaults in the database directly - Django never sets database
defaults and always applies them in the Django ORM code.

.. warning::

    On older databases, adding a field with a default value may cause a full
    rewrite of the table. This happens even for nullable fields and may have a
    negative performance impact. To avoid that, the following steps should be
    taken.

    * Add the nullable field without the default value and run the
      :djadmin:`makemigrations` command. This should generate a migration with
      an ``AddField`` operation.

    * Add the default value to your field and run the :djadmin:`makemigrations`
      command. This should generate a migration with an ``AlterField``
      operation.

``RemoveField``
---------------

.. class:: RemoveField(model_name, name)

Removes a field from a model.

Bear in mind that when reversed, this is actually adding a field to a model.
The operation is reversible (apart from any data loss, which is irreversible)
if the field is nullable or if it has a default value that can be used to
populate the recreated column. If the field is not nullable and does not have a
default value, the operation is irreversible.

``AlterField``
--------------

.. class:: AlterField(model_name, name, field, preserve_default=True)

Alters a field's definition, including changes to its type,
:attr:`~django.db.models.Field.null`, :attr:`~django.db.models.Field.unique`,
:attr:`~django.db.models.Field.db_column` and other field attributes.

The ``preserve_default`` argument indicates whether the field's default
value is permanent and should be baked into the project state (``True``),
or if it is temporary and just for this migration (``False``) - usually
because the migration is altering a nullable field to a non-nullable one and
needs a default value to put into existing rows. It does not affect the
behavior of setting defaults in the database directly - Django never sets
database defaults and always applies them in the Django ORM code.

Note that not all changes are possible on all databases - for example, you
cannot change a text-type field like ``models.TextField()`` into a number-type
field like ``models.IntegerField()`` on most databases.

``RenameField``
---------------

.. class:: RenameField(model_name, old_name, new_name)

Changes a field's name (and, unless :attr:`~django.db.models.Field.db_column`
is set, its column name).

``AddIndex``
------------

.. class:: AddIndex(model_name, index)

Creates an index in the database table for the model with ``model_name``.
``index`` is an instance of the :class:`~django.db.models.Index` class.

``RemoveIndex``
---------------

.. class:: RemoveIndex(model_name, name)

Removes the index named ``name`` from the model with ``model_name``.

``AddConstraint``
-----------------

.. class:: AddConstraint(model_name, constraint)

Creates a :doc:`constraint </ref/models/constraints>` in the database table for
the model with ``model_name``.

``RemoveConstraint``
--------------------

.. class:: RemoveConstraint(model_name, name)

Removes the constraint named ``name`` from the model with ``model_name``.

Special Operations
==================

``RunSQL``
----------

.. class:: RunSQL(sql, reverse_sql=None, state_operations=None, hints=None, elidable=False)

Allows running of arbitrary SQL on the database - useful for more advanced
features of database backends that Django doesn't support directly.

``sql``, and ``reverse_sql`` if provided, should be strings of SQL to run on
the database. On most database backends (all but PostgreSQL), Django will
split the SQL into individual statements prior to executing them.

.. warning::

    On PostgreSQL and SQLite, only use ``BEGIN`` or ``COMMIT`` in your SQL in
    :ref:`non-atomic migrations <non-atomic-migrations>`, to avoid breaking
    Django's transaction state.

You can also pass a list of strings or 2-tuples. The latter is used for passing
queries and parameters in the same way as :ref:`cursor.execute()
<executing-custom-sql>`. These three operations are equivalent::

    migrations.RunSQL("INSERT INTO musician (name) VALUES ('Reinhardt');")
    migrations.RunSQL([("INSERT INTO musician (name) VALUES ('Reinhardt');", None)])
    migrations.RunSQL([("INSERT INTO musician (name) VALUES (%s);", ['Reinhardt'])])

If you want to include literal percent signs in the query, you have to double
them if you are passing parameters.

The ``reverse_sql`` queries are executed when the migration is unapplied. They
should undo what is done by the ``sql`` queries. For example, to undo the above
insertion with a deletion::

    migrations.RunSQL(
        sql=[("INSERT INTO musician (name) VALUES (%s);", ['Reinhardt'])],
        reverse_sql=[("DELETE FROM musician where name=%s;", ['Reinhardt'])],
    )

If ``reverse_sql`` is ``None`` (the default), the ``RunSQL`` operation is
irreversible.

The ``state_operations`` argument allows you to supply operations that are
equivalent to the SQL in terms of project state. For example, if you are
manually creating a column, you should pass in a list containing an ``AddField``
operation here so that the autodetector still has an up-to-date state of the
model. If you don't, when you next run ``makemigrations``, it won't see any
operation that adds that field and so will try to run it again. For example::

    migrations.RunSQL(
        "ALTER TABLE musician ADD COLUMN name varchar(255) NOT NULL;",
        state_operations=[
            migrations.AddField(
                'musician',
                'name',
                models.CharField(max_length=255),
            ),
        ],
    )

The optional ``hints`` argument will be passed as ``**hints`` to the
:meth:`allow_migrate` method of database routers to assist them in making
routing decisions. See :ref:`topics-db-multi-db-hints` for more details on
database hints.

The optional ``elidable`` argument determines whether or not the operation will
be removed (elided) when :ref:`squashing migrations <migration-squashing>`.

.. attribute:: RunSQL.noop

    Pass the ``RunSQL.noop`` attribute to ``sql`` or ``reverse_sql`` when you
    want the operation not to do anything in the given direction. This is
    especially useful in making the operation reversible.

``RunPython``
-------------

.. class:: RunPython(code, reverse_code=None, atomic=None, hints=None, elidable=False)

Runs custom Python code in a historical context. ``code`` (and ``reverse_code``
if supplied) should be callable objects that accept two arguments; the first is
an instance of ``django.apps.registry.Apps`` containing historical models that
match the operation's place in the project history, and the second is an
instance of :class:`SchemaEditor
<django.db.backends.base.schema.BaseDatabaseSchemaEditor>`.

The ``reverse_code`` argument is called when unapplying migrations. This
callable should undo what is done in the ``code`` callable so that the
migration is reversible. If ``reverse_code`` is ``None`` (the default), the
``RunPython`` operation is irreversible.

The optional ``hints`` argument will be passed as ``**hints`` to the
:meth:`allow_migrate` method of database routers to assist them in making a
routing decision. See :ref:`topics-db-multi-db-hints` for more details on
database hints.

The optional ``elidable`` argument determines whether or not the operation will
be removed (elided) when :ref:`squashing migrations <migration-squashing>`.

You are advised to write the code as a separate function above the ``Migration``
class in the migration file, and pass it to ``RunPython``. Here's an example of
using ``RunPython`` to create some initial objects on a ``Country`` model::

    from django.db import migrations

    def forwards_func(apps, schema_editor):
        # We get the model from the versioned app registry;
        # if we directly import it, it'll be the wrong version
        Country = apps.get_model("myapp", "Country")
        db_alias = schema_editor.connection.alias
        Country.objects.using(db_alias).bulk_create([
            Country(name="USA", code="us"),
            Country(name="France", code="fr"),
        ])

    def reverse_func(apps, schema_editor):
        # forwards_func() creates two Country instances,
        # so reverse_func() should delete them.
        Country = apps.get_model("myapp", "Country")
        db_alias = schema_editor.connection.alias
        Country.objects.using(db_alias).filter(name="USA", code="us").delete()
        Country.objects.using(db_alias).filter(name="France", code="fr").delete()

    class Migration(migrations.Migration):

        dependencies = []

        operations = [
            migrations.RunPython(forwards_func, reverse_func),
        ]

This is generally the operation you would use to create
:ref:`data migrations <data-migrations>`, run
custom data updates and alterations, and anything else you need access to an
ORM and/or Python code for.

Much like :class:`RunSQL`, ensure that if you change schema inside here you're
either doing it outside the scope of the Django model system (e.g. triggers)
or that you use :class:`SeparateDatabaseAndState` to add in operations that will
reflect your changes to the model state - otherwise, the versioned ORM and
the autodetector will stop working correctly.

By default, ``RunPython`` will run its contents inside a transaction on
databases that do not support DDL transactions (for example, MySQL and
Oracle). This should be safe, but may cause a crash if you attempt to use
the ``schema_editor`` provided on these backends; in this case, pass
``atomic=False`` to the ``RunPython`` operation.

On databases that do support DDL transactions (SQLite and PostgreSQL),
``RunPython`` operations do not have any transactions automatically added
besides the transactions created for each migration. Thus, on PostgreSQL, for
example, you should avoid combining schema changes and ``RunPython`` operations
in the same migration or you may hit errors like ``OperationalError: cannot
ALTER TABLE "mytable" because it has pending trigger events``.

If you have a different database and aren't sure if it supports DDL
transactions, check the ``django.db.connection.features.can_rollback_ddl``
attribute.

If the ``RunPython`` operation is part of a :ref:`non-atomic migration
<non-atomic-migrations>`, the operation will only be executed in a transaction
if ``atomic=True`` is passed to the ``RunPython`` operation.

.. warning::

    ``RunPython`` does not magically alter the connection of the models for you;
    any model methods you call will go to the default database unless you
    give them the current database alias (available from
    ``schema_editor.connection.alias``, where ``schema_editor`` is the second
    argument to your function).

.. staticmethod:: RunPython.noop

    Pass the ``RunPython.noop`` method to ``code`` or ``reverse_code`` when
    you want the operation not to do anything in the given direction. This is
    especially useful in making the operation reversible.

``SeparateDatabaseAndState``
----------------------------

.. class:: SeparateDatabaseAndState(database_operations=None, state_operations=None)

A highly specialized operation that lets you mix and match the database
(schema-changing) and state (autodetector-powering) aspects of operations.

It accepts two lists of operations. When asked to apply state, it will use the
``state_operations`` list (this is a generalized version of :class:`RunSQL`'s
``state_operations`` argument). When asked to apply changes to the database, it
will use the ``database_operations`` list.

If the actual state of the database and Django's view of the state get out of
sync, this can break the migration framework, even leading to data loss. It's
worth exercising caution and checking your database and state operations
carefully. You can use :djadmin:`sqlmigrate` and :djadmin:`dbshell` to check
your database operations. You can use :djadmin:`makemigrations`, especially
with :option:`--dry-run<makemigrations --dry-run>`, to check your state
operations.

For an example using ``SeparateDatabaseAndState``, see
:ref:`changing-a-manytomanyfield-to-use-a-through-model`.

.. _writing-your-own-migration-operation:

Writing your own
================

Operations have a relatively simple API, and they're designed so that you can
easily write your own to supplement the built-in Django ones. The basic
structure of an ``Operation`` looks like this::

    from django.db.migrations.operations.base import Operation

    class MyCustomOperation(Operation):

        # If this is False, it means that this operation will be ignored by
        # sqlmigrate; if true, it will be run and the SQL collected for its output.
        reduces_to_sql = False

        # If this is False, Django will refuse to reverse past this operation.
        reversible = False

        def __init__(self, arg1, arg2):
            # Operations are usually instantiated with arguments in migration
            # files. Store the values of them on self for later use.
            pass

        def state_forwards(self, app_label, state):
            # The Operation should take the 'state' parameter (an instance of
            # django.db.migrations.state.ProjectState) and mutate it to match
            # any schema changes that have occurred.
            pass

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            # The Operation should use schema_editor to apply any changes it
            # wants to make to the database.
            pass

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            # If reversible is True, this is called when the operation is reversed.
            pass

        def describe(self):
            # This is used to describe what the operation does in console output.
            return "Custom Operation"

        @property
        def migration_name_fragment(self):
            # Optional. A filename part suitable for automatically naming a
            # migration containing this operation, or None if not applicable.
            return "custom_operation_%s_%s" % (self.arg1, self.arg2)

You can take this template and work from it, though we suggest looking at the
built-in Django operations in ``django.db.migrations.operations`` - they cover
a lot of the example usage of semi-internal aspects of the migration framework
like ``ProjectState`` and the patterns used to get historical models, as well
as ``ModelState`` and the patterns used to mutate historical models in
``state_forwards()``.

Some things to note:

* You don't need to learn too much about ``ProjectState`` to write migrations;
  just know that it has an ``apps`` property that gives access to an app
  registry (which you can then call ``get_model`` on).

* ``database_forwards`` and ``database_backwards`` both get two states passed
  to them; these represent the difference the ``state_forwards`` method would
  have applied, but are given to you for convenience and speed reasons.

* If you want to work with model classes or model instances from the
  ``from_state`` argument in ``database_forwards()`` or
  ``database_backwards()``, you must render model states using the
  ``clear_delayed_apps_cache()`` method to make related models available::

    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        # This operation should have access to all models. Ensure that all models are
        # reloaded in case any are delayed.
        from_state.clear_delayed_apps_cache()
        ...

* ``to_state`` in the database_backwards method is the *older* state; that is,
  the one that will be the current state once the migration has finished reversing.

* You might see implementations of ``references_model`` on the built-in
  operations; this is part of the autodetection code and does not matter for
  custom operations.

.. warning::

    For performance reasons, the :class:`~django.db.models.Field` instances in
    ``ModelState.fields`` are reused across migrations. You must never change
    the attributes on these instances. If you need to mutate a field in
    ``state_forwards()``, you must remove the old instance from
    ``ModelState.fields`` and add a new instance in its place. The same is true
    for the :class:`~django.db.models.Manager` instances in
    ``ModelState.managers``.

As an example, let's make an operation that loads PostgreSQL extensions (which
contain some of PostgreSQL's more exciting features). Since there's no model
state changes, all it does is run one command::

    from django.db.migrations.operations.base import Operation

    class LoadExtension(Operation):

        reversible = True

        def __init__(self, name):
            self.name = name

        def state_forwards(self, app_label, state):
            pass

        def database_forwards(self, app_label, schema_editor, from_state, to_state):
            schema_editor.execute("CREATE EXTENSION IF NOT EXISTS %s" % self.name)

        def database_backwards(self, app_label, schema_editor, from_state, to_state):
            schema_editor.execute("DROP EXTENSION %s" % self.name)

        def describe(self):
            return "Creates extension %s" % self.name

        @property
        def migration_name_fragment(self):
            return "create_extension_%s" % self.name
