ContentType.objects.get_for_models() in migrations does not works for multiple models

[HMaker]

I am trying to use migrations to create default groups, I tried to run the following procedure with migrations.RunPython()

def create_normal_users_group(apps, *args):
    auth = SimpleNamespace(**apps.all_models['auth'])
    myapp = SimpleNamespace(**apps.all_models['myapp'])
    ContentType = apps.get_model('contenttypes', 'ContentType')
    group = auth.group.objects.create(name='Normal Users')
    contenttypes = ContentType.objects.get_for_models(myapp.user, myapp.proxy) # there are more models...
   # ...

but it raises AttributeError

  File "/home/user/projects/myapp/.venv/lib/python3.8/site-packages/django/contrib/contenttypes/models.py", line 89, in get_for_models
    opts_models = needed_opts.pop(ct.model_class()._meta, [])
AttributeError: 'ContentType' object has no attribute 'model_class'

it works when I pass a single model to ContentType.objects.get_for_models(), but get_for_models() is supposed to work with multiple models.

EDIT:
Adding model_class() to ContentType makes it work

def create_normal_users_group(apps, *args):
    auth = SimpleNamespace(**apps.all_models['auth'])
    myapp = SimpleNamespace(**apps.all_models['myapp'])
    ContentType = apps.get_model('contenttypes', 'ContentType')
    # this makes it work =========================
    def model_class(self):
        return apps.get_model(self.app_label, self.model)
    ContentType.model_class = model_class
    # ========================================
    group = auth.group.objects.create(name='Normal Users')
    contenttypes = ContentType.objects.get_for_models(myapp.user, myapp.proxy) # there are more models...
   # ...

[Simon Charette]

Since ContentTypeManager is marked as use_in_migrations = True it should be able to handle this case.

There's two possible solutions here:

1. Create an AbstractContentType(models.Model) abstract model class that include most of the logic ContentType and have the latter subclass the former. From there CreateModel(bases) ​should be adjusted to pass this base instead.
2. Adjust ContentTypeManager.get_for_models to avoid calling .model_class() and inline its logic in the manager method instead.

Feels like 2. is the less disruptive approach and easier to test. Would you be interested in writing a patch PR for it?

[HMaker]

OK I will implement 2nd solution.

[HMaker]

I tried to reproduce this problem but I could not, check my ​partial patch, it's based on 3.1.4 branch.

[Keryn Knight]

Replying to Heraldo Lucena:

I tried to reproduce this problem but I could not [...]

My guess would be that the ContentTypeManager instance already had a partially filled (or empty) cache when you encountered the error, such that self._get_from_cache(modelinstance._meta) worked (didn't throw a KeyError looking in the self._cache).

• If you pass in 2 model classes Group and User and both are already in its internal cache, then needed_opts is empty and needed_opts.pop(ct.model_class()._meta, []) doesn't occur.
• If you pass the same 2 classes and only 1 or 0 of them are in the self._cache already, you'd fill the missing items into needed_opts and hit the line which has caused you problems.

[HMaker]

I thought migrations would be automatically rolled back after each test case, but that did not happen and last test case could not run properly since all migrations were already applied.

[GitHub <noreply@…>]

In 84206607:

Fixed #32833 -- Fixed ContentTypeManager.get_for_models() crash when using in migrations.

Co-authored-by: Heraldo Lucena <23155511+HMaker@…>

[Sarah Boyce <42296566+sarahboyce@…>]

In f1705c8:

Fixed #35545, Refs #32833 -- Fixed ContentTypeManager.get_for_models() crash in CreateModel migrations.

Thank you to Csirmaz Bendegúz for the report and Simon Charettes for the review.

[Sarah Boyce <42296566+sarahboyce@…>]

In 6317803:

[5.1.x] Fixed #35545, Refs #32833 -- Fixed ContentTypeManager.get_for_models() crash in CreateModel migrations.

Thank you to Csirmaz Bendegúz for the report and Simon Charettes for the review.

Backport of f1705c8780c0a7587654fc736542d55fe4a7f29b from main.