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.

Accessing models from other apps

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

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()``.

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 = [

            ('app1', '0001_initial'),

            # added dependency to enable using models from app2 in move_m1

            ('app2', '0004_foobar'),

        ]

        operations = [

            migrations.RunPython(move_m1),

        ]

More advanced migrations

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