Unapplying a migration has side-effects on the ProjectState passed to the method

[Markus Holtermann]

django.db.migrations.migration.Migration.unapply() accepts an argument state that represents the project state right before the migration is applied. The current implementation alters this state when building the intermediate states to which each operation rolls back, instead of using a copy. This side effect results in errors where e.g. a model can exists in the state, but its gone from the database.

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 73, in migrate
    state = self.unapply_migration(state, migration, fake=fake)
  File "/home/markus/Coding/django/django/db/migrations/executor.py", line 127, in unapply_migration
    state = migration.unapply(state, schema_editor)
  File "/home/markus/Coding/django/django/db/migrations/migration.py", line 135, in unapply
    operation.state_forwards(self.app_label, project_state)
  File "/home/markus/Coding/django/django/db/migrations/operations/models.py", line 53, in state_forwards
    list(self.managers),
  File "/home/markus/Coding/django/django/db/migrations/state.py", line 39, in add_model
    self.reload_model(app_label, model_name)
  File "/home/markus/Coding/django/django/db/migrations/state.py", line 61, in reload_model
    self._reload_one_model(rel_model._meta.app_label, rel_model._meta.model_name)
AttributeError: 'str' object has no attribute '_meta'

[Markus Holtermann]

It looks like not all _pending_lookups in the StateApps get resolved.

[Markus Holtermann]

PR ​https://github.com/django/django/pull/3881

#24099 relies on a correct behavior of Migration.unapply()

[Markus Holtermann <info@…>]

In fdc2cc948725866212a9bcc97b9b7cf21bb49b90:

Fixed #24110 -- Rewrote migration unapply to preserve intermediate states

[Markus Holtermann <info@…>]

In be158e36251df0b07556657da47cdaf10913c57a:

Refs #24110 -- Added a more descriptive release note and fixed a spelling mistake.

[Markus Holtermann <info@…>]

In ef5889409bba675499dfd38831358daba16811ab:

[1.7.x] Fixed #24110 -- Rewrote migration unapply to preserve intermediate states

Backport of fdc2cc948725866212a9bcc97b9b7cf21bb49b90 and be158e36251df0b07556657da47cdaf10913c57a from master