MigrationOptimizer mangles operation order if app name contains uppercase letters.

[Koen De Wit]

I am aware that app names are preferably all-lowercase according to ​PEP 8, but uppercase letters are nevertheless valid.

Steps to reproduce:

• Create a new project and an app with uppercase letters in the app name :

  django-admin startproject mysite
  cd mysite
  python manage.py startapp MyApp
  

• Add 'MyApp' to the INSTALLED_APPS in mysite/settings.py
• Edit MyApp/models.py :

  from django.db import models
  
  class RefModel(models.Model):
      pass
  
  class BaseModel(models.Model):
      r = models.ForeignKey(RefModel, on_delete=models.PROTECT)
  
  class SubModel(BaseModel):
      pass
  

• Run python ./manage.py makemigrations . In the resulting migration file, the create operation for SubModel comes before the create operation for BaseModel, which is wrong.
• Run python ./manage.py migrate , which will fail with this error:

  Operations to perform:
    Apply all migrations: MyApp, admin, auth, contenttypes, sessions
  Running migrations:
    Applying MyApp.0001_initial...Traceback (most recent call last):
    File "./manage.py", line 22, in <module>
      main()
    File "./manage.py", line 18, in main
      execute_from_command_line(sys.argv)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
      utility.execute()
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/core/management/__init__.py", line 395, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/core/management/base.py", line 330, in run_from_argv
      self.execute(*args, **cmd_options)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/core/management/base.py", line 371, in execute
      output = self.handle(*args, **options)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/core/management/base.py", line 85, in wrapped
      res = handle_func(*args, **kwargs)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/core/management/commands/migrate.py", line 243, in handle
      post_migrate_state = executor.migrate(
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/executor.py", line 117, in migrate
      state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
      state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/executor.py", line 227, in apply_migration
      state = migration.apply(state, schema_editor)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/migration.py", line 114, in apply
      operation.state_forwards(self.app_label, project_state)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/operations/models.py", line 80, in state_forwards
      state.add_model(ModelState(
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/state.py", line 95, in add_model
      self.reload_model(app_label, model_name)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/state.py", line 156, in reload_model
      self._reload(related_models)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/state.py", line 189, in _reload
      self.apps.render_multiple(states_to_be_rendered)
    File "/home/koen/.virtualenvs/dtest/lib/python3.8/site-packages/django/db/migrations/state.py", line 310, in render_multiple
      raise InvalidBasesError(
  django.db.migrations.exceptions.InvalidBasesError: Cannot resolve bases for [<ModelState: 'MyApp.SubModel'>]
  This can happen if you are inheriting models from an app with migrations (e.g. contrib.auth)
   in an app with no migrations; see https://docs.djangoproject.com/en/3.1/topics/migrations/#dependencies for more
  

This bug does not occur if the app name is all-lowercase.

Digging into the code, I found that the MigrationOptimizer will combine two operations (Create model BaseModel and add ForeignKey-field r to BaseModel) without taking into account that SubModel depends on BaseModel.

[Mariusz Felisiak]

​PR

[Koen De Wit]

Added test: ​https://github.com/django/django/pull/13354/files

[Mariusz Felisiak <felisiak.mariusz@…>]

In 4c0b472:

Fixed #31954 -- Fixed migration optimization for MTI model creation with parent model with mixed case app label.