Fixed #28056 - Reverse migration for model rename with cross-app ForeignKey fails

[gaurav8297]

During the renaming of model. To maintain the migration state of related field it is necessary to include AlterField migration.

Ticket: https://code.djangoproject.com/ticket/28056

[timgraham]

Did you test with the sample project on the ticket? After regenerating the migration and migrating forwards, I'm seeing this error:

Running migrations:
  Applying one.0001_initial... OK
  Applying one.0002_auto_20170616_1919... OK
  Applying two.0001_initial...Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/home/tim/code/django/django/core/management/__init__.py", line 354, in execute_from_command_line
    utility.execute()
  File "/home/tim/code/django/django/core/management/__init__.py", line 348, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/tim/code/django/django/core/management/base.py", line 280, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/tim/code/django/django/core/management/base.py", line 327, in execute
    output = self.handle(*args, **options)
  File "/home/tim/code/django/django/core/management/commands/migrate.py", line 200, in handle
    fake_initial=fake_initial,
  File "/home/tim/code/django/django/db/migrations/executor.py", line 113, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/home/tim/code/django/django/db/migrations/executor.py", line 143, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/home/tim/code/django/django/db/migrations/executor.py", line 240, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/tim/code/django/django/db/migrations/migration.py", line 122, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/tim/code/django/django/db/migrations/operations/models.py", line 92, in database_forwards
    schema_editor.create_model(model)
  File "/home/tim/code/django/django/db/backends/base/schema.py", line 242, in create_model
    definition, extra_params = self.column_sql(model, field)
  File "/home/tim/code/django/django/db/backends/base/schema.py", line 134, in column_sql
    db_params = field.db_parameters(connection=self.connection)
  File "/home/tim/code/django/django/db/models/fields/related.py", line 934, in db_parameters
    return {"type": self.db_type(connection), "check": self.db_check(connection)}
  File "/home/tim/code/django/django/db/models/fields/related.py", line 931, in db_type
    return self.target_field.rel_db_type(connection=connection)
  File "/home/tim/code/django/django/db/models/fields/related.py", line 852, in target_field
    return self.foreign_related_fields[0]
  File "/home/tim/code/django/django/db/models/fields/related.py", line 624, in foreign_related_fields
    return tuple(rhs_field for lhs_field, rhs_field in self.related_fields if rhs_field)
  File "/home/tim/code/django/django/db/models/fields/related.py", line 611, in related_fields
    self._related_fields = self.resolve_related_fields()
  File "/home/tim/code/django/django/db/models/fields/related.py", line 596, in resolve_related_fields
    raise ValueError('Related model %r cannot be resolved' % self.remote_field.model)
ValueError: Related model 'one.Rename1' cannot be resolved

As I mentioned on the ticket, I think the problem is in ForeignKey.db_type(). It uses self.target_field which is using the state of the current model files rather than migration's state. I'm not sure if this is fixable at the auto-detector level. Any insight @andrewgodwin?

[gaurav8297]

@timgraham sorry for that when i tested i did migration and migrating first and after renaming model i did these two steps again and it worked for me. But now i think i have to check it again. I will try to resolve this and get back to you soon.

[timgraham]

I guess I'll close it for now.