Migrations only work for apps containing models

[David Seddon]

Migrations only work for apps containing a models.py. This means that if you create a data migration (./manage.py makemigrations --empty <app>) for an app without a models.py, it will fail to execute when running ./manage.py migrate.

I think it would be better if apps containing migrations would run, regardless of having that empty file.

This is because it can be a nice structure to have a 'main' app responsible for general project-wide stuff, which includes running data migrations that aren't directly related to models. An example might be assigning some permissions in the database to a particular role, but for a third party app.

Not sure if this is the only place where the check is made, but django.db.migrations.loader.MigrationLoader.load_disk() uses the following test to see whether to include the app:

if app_config.models_module is None:
    continue

[Tim Graham]

See also #22645 - "GIS tests break without a models.py`. We removed that check and then something else broke (I forget what) so it had to be reverted.

[Claude Paroz]

Typical GIS error (whatever the backend) when removing the app_config.models_module check:

Applying gis_migrations.0001_initial...Traceback (most recent call last):
  File "./runtests.py", line 391, in <module>
    options.failfast, options.modules)
  File "./runtests.py", line 234, in django_tests
    test_labels or get_installed(), extra_tests=extra_tests)
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/test/runner.py", line 151, in run_tests
    old_config = self.setup_databases()
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/test/runner.py", line 113, in setup_databases
    return setup_databases(self.verbosity, self.interactive, self.keepdb, **kwargs)
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/test/runner.py", line 304, in setup_databases
    serialize=connection.settings_dict.get("TEST", {}).get("SERIALIZE", True),
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/db/backends/creation.py", line 385, in create_test_db
    test_flush=True,
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/core/management/__init__.py", line 118, in call_command
    return command.execute(*args, **defaults)
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/core/management/base.py", line 419, in execute
    output = self.handle(*args, **options)
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/core/management/commands/migrate.py", line 193, in handle
    executor.migrate(targets, plan, fake=options.get("fake", False))
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/db/migrations/executor.py", line 63, in migrate
    self.apply_migration(migration, fake=fake)
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/db/migrations/executor.py", line 91, in apply_migration
    if self.detect_soft_applied(migration):
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/db/migrations/executor.py", line 135, in detect_soft_applied
    apps = project_state.render()
  File "/home/jenkins/workspace/django-pull-requests/database/mysql_gis/python/python2.7/django/db/migrations/state.py", line 89, in render
    model=lookup_model,
ValueError: Lookup failed for model referenced by field gis_migrations.Household.neighborhood: gis.Neighborhood

​http://djangoci.com/job/django-pull-requests/1249/

[Claude Paroz]

I've update ​https://github.com/django/django/pull/3340.
At least the test suite now starts, but strangely, the test suite passes when I only run it with django.contrib.gis test label, but fails with no test labels.

[Claude Paroz]

I think I resolved the test-order-related failures.
I guess this warrants a release note.

[Claude Paroz <claude@…>]

In e50e0ee27bf16090fe36a87cf91fe2baa27b70f1:

Fixed #23618 -- Allowed apps with no models to still have migrations

Basically a4737bf6ae reapplied.