[1.7.x] Fixed #24052 -- Doc'd how to write data migrations with models in multiple apps.

want executed when migrating backwards. If this callable is omitted, migrating

backwards will raise an exception.

.. _data-migrations-and-multiple-databases:

Accessing models from other apps

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Data migrations and multiple databases

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When writing a ``RunPython`` function that uses models from apps other than the

one in which the migration is located, the migration's ``dependencies``

attribute should include the latest migration of each app that is involved,

otherwise you may get an error similar to: ``LookupError: No installed app

with label 'myappname'`` when you try to retrieve the model in the ``RunPython``

function using ``apps.get_model()``.

When using multiple databases, you may need to figure out whether or not to

run a migration against a particular database. For example, you may want to

**only** run a migration on a particular database.

In order to do that you can check the database connection's alias inside a

``RunPython`` operation by looking at the ``schema_editor.connection.alias``

attribute::

    from django.db import migrations

    def forwards(apps, schema_editor):

        if not schema_editor.connection.alias == 'default':

            return

        # Your migration code goes here

    class Migration(migrations.Migration):

        dependencies = [

            # Dependencies to other migrations

        ]

        operations = [

            migrations.RunPython(forwards),

        ]

You can also use your database router's ``allow_migrate()`` method, but keep in

mind that the imported router needs to stay around as long as it is referenced

inside a migration:

.. snippet::

    :filename: myapp/dbrouters.py

    class MyRouter(object):

        def allow_migrate(self, db, model):

            return db == 'default'

Then, to leverage this in your migrations, do the following::

    from django.db import migrations

    from myappname.dbrouters import MyRouter

    def forwards(apps, schema_editor):

        MyModel = apps.get_model("myappname", "MyModel")

        if not MyRouter().allow_migrate(schema_editor.connection.alias, MyModel):

            return

        # Your migration code goes here

In the following example, we have a migration in ``app1`` which needs to use

models in ``app2``. We aren't concerned with the details of ``move_m1`` other

than the fact it will need to access models from both apps. Therefore we've

added a dependency that specifies the last migration of ``app2``::

    class Migration(migrations.Migration):

        dependencies = [

            # Dependencies to other migrations

            ('app1', '0001_initial'),

            # added dependency to enable using models from app2 in move_m1

            ('app2', '0004_foobar'),

        ]

        operations = [

            migrations.RunPython(forwards),

            migrations.RunPython(move_m1),

        ]

More advanced migrations