mirror of https://github.com/django/django.git
Fixed #24016 -- Added documentation about third-party app data migrations
There was confusion about how to migrate data from third-party applications when you are going to uninstall the application later on. Thanks to Markus, Marten and Sergei for help and review.
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@ -713,6 +713,7 @@ answer newbie questions, and generally made Django that much better:
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Tim Graham <timograham@gmail.com>
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Tim Heap <tim@timheap.me>
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Tim Saylor <tim.saylor@gmail.com>
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Tobias Kunze <rixx@cutebit.de>
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Tobias McNulty <http://www.caktusgroup.com/blog>
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tobias@neuyork.de
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Todd O'Bryan <toddobryan@mac.com>
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@ -271,3 +271,64 @@ Prefer using ``dependencies`` over ``run_before`` when possible. You should
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only use ``run_before`` if it is undesirable or impractical to specify
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``dependencies`` in the migration which you want to run after the one you are
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writing.
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Migrating data when replacing an external app
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=============================================
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If you plan to move from one external application to another one with a similar
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data structure, you can use a data migration. If you plan to remove the old
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application later, you will need to set the ``dependencies`` property
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dynamically. Otherwise you will have missing dependencies once you uninstall
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the old application.
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.. snippet::
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:filename: myapp/migrations/0124_ensure_dependencies.py
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from django.apps import apps as global_apps
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from django.db import migrations
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def forward(apps, schema_editor):
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"""
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see below
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"""
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class Migration(migrations.Migration):
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operations = [
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migrations.RunPython(forward, migrations.RunPython.noop),
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]
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dependencies = [
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('myapp', '0123_the_previous_migration'),
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('new_external_app', '0001_initial'),
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]
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if global_apps.is_installed('old_external_app'):
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dependencies.append(('old_external_app', '0001_initial'))
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In your data migration method, you will need to test for the old application
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model:
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.. snippet::
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:filename: myapp/migrations/0124_ensure_dependencies.py
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def forward(apps, schema_editor):
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try:
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OldModel = apps.get_model('old_external', 'OldModel')
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except LookupError:
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return
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NewModel = apps.get_model('new_external', 'NewModel')
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NewModel.objects.bulk_create(
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NewModel(new_attribute=old_object.old_attribute)
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for old_object in OldModel.objects.all()
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)
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This way you can deploy your application anywhere without first installing
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and then uninstalling your old external dependency. If the old external
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dependency is not installed when the migration runs it will just do nothing
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instead of migrating the data.
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Please take also into consideration what you want to happen when the migration
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is unapplied - you could either do nothing or remove some or all data from
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the new application model; adjust the second argument of the
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:mod:`~django.db.migrations.operations.RunPython` operation accordingly.
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