LookupError when rolling back migrations

[Markus Holtermann]

The MigrationExecutor.migrate() method generates the initial project state from the first migration that is being applied/unapplied. If this migration has no relations to other apps that are also being (un)applied, this can lead to LookupErrors due to missing models:

Traceback (most recent call last):
  File "/home/markus/Coding/django/django/apps/registry.py", line 148, in get_app_config
    return self.app_configs[app_label]
KeyError: 'app_b'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/markus/Coding/django/django/db/migrations/state.py", line 181, in __init__
    model = self.get_model(lookup_model[0], lookup_model[1])
  File "/home/markus/Coding/django/django/apps/registry.py", line 202, in get_model
    return self.get_app_config(app_label).get_model(model_name.lower())
  File "/home/markus/Coding/django/django/apps/registry.py", line 150, in get_app_config
    raise LookupError("No installed app with label '%s'." % app_label)
LookupError: No installed app with label 'app_b'.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/markus/Coding/django/django/core/management/__init__.py", line 338, in execute_from_command_line
    utility.execute()
  File "/home/markus/Coding/django/django/core/management/__init__.py", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/markus/Coding/django/django/core/management/base.py", line 390, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/markus/Coding/django/django/core/management/base.py", line 444, in execute
    output = self.handle(*args, **options)
  File "/home/markus/Coding/django/django/core/management/commands/migrate.py", line 213, in handle
    executor.migrate(targets, plan, fake=options.get("fake", False))
  File "/home/markus/Coding/django/django/db/migrations/executor.py", line 75, in migrate
    state = self.unapply_migration(state, migration, fake=fake)
  File "/home/markus/Coding/django/django/db/migrations/executor.py", line 130, in unapply_migration
    state = migration.unapply(state, schema_editor)
  File "/home/markus/Coding/django/django/db/migrations/migration.py", line 157, in unapply
    operation.database_backwards(self.app_label, schema_editor, from_state, to_state)
  File "/home/markus/Coding/django/django/db/migrations/operations/models.py", line 62, in database_backwards
    model = from_state.apps.get_model(app_label, self.name)
  File "/home/markus/Coding/django/django/utils/functional.py", line 60, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/home/markus/Coding/django/django/db/migrations/state.py", line 82, in apps
    return StateApps(self.real_apps, self.models)
  File "/home/markus/Coding/django/django/db/migrations/state.py", line 191, in __init__
    raise ValueError(msg.format(field=operations[0][1], model=lookup_model))
ValueError: Lookup failed for model referenced by field app_a.A3.b2: app_b.B2

Given the attached migration dependencies, the backwards plan for ('app_a', None) is

[
    (<Migration app_c.0003_c3>, True),
    (<Migration app_a.0004_a4>, True),
    (<Migration app_a.0003_a3>, True),
    (<Migration app_c.0002_c2>, True),
    (<Migration app_b.0003_b3>, True),
    (<Migration app_b.0002_b2>, True),
    (<Migration app_a.0002_a2>, True),
    (<Migration app_a.0001_initial>, True)
]

The initial state generated from app_c.0003_c3 is

  app_a.a1:
  app_a.a2:
  app_c.c1:
  app_c.c2:

As one can see, there are no models of app_b, which eventually leads to an error when unapplying app_1.0003_a3 which tries to look up model app_b.B2.