Refs #24016 -- Edited "Migrating data between third-party apps" howto.

``dependencies`` in the migration which you want to run after the one you are

writing.

Migrating data when replacing an external app

=============================================

Migrating data between third-party apps

=======================================

If you plan to move from one external application to another one with a similar

data structure, you can use a data migration. If you plan to remove the old 

application later, you will need to set the ``dependencies`` property

dynamically. Otherwise you will have missing dependencies once you uninstall

the old application.

You can use a data migration to move data from one third-party application to

another.

If you plan to remove the old app later, you'll need to set the ``dependencies``

property based on whether or not the old app is installed. Otherwise, you'll

have missing dependencies once you uninstall the old app. Similarly, you'll

need to catch :exc:`LookupError` in the ``apps.get_model()`` call that

retrieves models from the old app. This approach allows you to deploy your

project anywhere without first installing and then uninstalling the old app.

Here's a sample migration:

.. snippet::

    :filename: myapp/migrations/0124_ensure_dependencies.py

    :filename: myapp/migrations/0124_move_old_app_to_new_app.py

    from django.apps import apps as global_apps

    from django.db import migrations

    def forward(apps, schema_editor):

        """

        see below

        """

    def forwards(apps, schema_editor):

        try:

            OldModel = apps.get_model('old_app', 'OldModel')

        except LookupError:

            # The old app isn't installed.

            return

        NewModel = apps.get_model('new_app', 'NewModel')

        NewModel.objects.bulk_create(

            NewModel(new_attribute=old_object.old_attribute)

            for old_object in OldModel.objects.all()

        )

    class Migration(migrations.Migration):

        operations = [

            migrations.RunPython(forward, migrations.RunPython.noop),

            migrations.RunPython(forwards, migrations.RunPython.noop),

        ]

        dependencies = [

            ('myapp', '0123_the_previous_migration'),

            ('new_external_app', '0001_initial'),

            ('new_app', '0001_initial'),

        ]

        if global_apps.is_installed('old_external_app'):

            dependencies.append(('old_external_app', '0001_initial'))

        if global_apps.is_installed('old_app'):

            dependencies.append(('old_app', '0001_initial'))

In your data migration method, you will need to test for the old application

model:

.. snippet::

    :filename: myapp/migrations/0124_ensure_dependencies.py

    def forward(apps, schema_editor):

        try:

            OldModel = apps.get_model('old_external', 'OldModel')

        except LookupError:

            return

        NewModel = apps.get_model('new_external', 'NewModel')

        NewModel.objects.bulk_create(

            NewModel(new_attribute=old_object.old_attribute)

            for old_object in OldModel.objects.all()

        )

This way you can deploy your application anywhere without first installing

and then uninstalling your old external dependency. If the old external

dependency is not installed when the migration runs it will just do nothing

instead of migrating the data.

Please take also into consideration what you want to happen when the migration

is unapplied - you could either do nothing or remove some or all data from

the new application model; adjust the second argument of the

Also consider what you want to happen when the migration is unapplied. You

could either do nothing (as in the example above) or remove some or all of the

data from the new application. Adjust the second argument of the

:mod:`~django.db.migrations.operations.RunPython` operation accordingly.