Model.<related_name> has no attribute "through"

[michjnich]

Bug report

What's wrong

I confess, I am quite new to typing, so if this is a mypy issue and not a django-stubs issue, I apologise. I think I am reporting this in the right place though.

I have a ManyToManyField on a model:

class Post(models.Model):
   ...
    identifiers = models.ManyToManyField(Identifier, verbose_name=_("Identifiers"), related_name="posts")

And a TabularInline in admin for the Identifier model:

class PostInline(admin.TabularInline):
    model = Identifier.posts.through
    ...

But mypy does not like this :

error: "RelatedManager[Post]" has no attribute "through"  [attr-defined]

How is that should be

No error should be raised because Identifier.posts.through is valid

System information

• OS: WSL2 under windows 11
  Linux LMAVSURFACE4 5.15.90.1-microsoft-standard-WSL2 #1 SMP Fri Jan 27 02:56:13 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
• python version: 3.11
• django version: 4.2.6
• mypy version: 1.6.1
• mypy-extensions version: 1.0.0
• django-stubs version: 4.2.5
• django-stubs-ext version: 4.2.5

[intgr]

Hi! Did you configure mypy to use the mypy_django_plugin.main plugin? And did you configure a functional django_settings_module?

[michjnich]

I have this in pyproject.toml

[tool.mypy]
python_version = "3.11"
plugins = ["mypy_django_plugin.main"]

[tool.django-stubs]
django_settings_module = "config.settings"

[intgr]

And does the INSTALLED_APPS in config.settings enable the app where your Post model exists?

[michjnich]

It does, yes. (otherwise pretty much nothing would work :) )

[flaeppe]

I can reproduce this in test_many_to_many

django-stubs/tests/typecheck/fields/test_related.yml

Lines 1070 to 1095 in 25e23fa

	- case: test_many_to_many
	main: |
	from myapp.models import MyModel, Other
	reveal_type(MyModel.auto_through.through.objects.get())
	reveal_type(MyModel().auto_through.get())
	reveal_type(Other().autos.get())
	reveal_type(MyModel.custom_through.through.objects.get())
	reveal_type(MyModel().custom_through.get())
	reveal_type(MyModel.custom_through.through.objects.custom_qs_method())
	reveal_type(Other().customs.get())
	auto_through = MyModel.auto_through.through.objects.get()
	reveal_type(auto_through.id)
	reveal_type(auto_through.pk)
	reveal_type(auto_through.mymodel)
	reveal_type(auto_through.mymodel_id)
	reveal_type(auto_through.other)
	reveal_type(auto_through.other_id)
	reveal_type(MyModel.auto_through.through)
	reveal_type(MyModel.auto_through.through.mymodel)
	reveal_type(MyModel.auto_through.through._default_manager)
	reveal_type(MyModel.other_again.through)
	out: |

 -   case: test_many_to_many
    main: |
        from myapp.models import MyModel, Other
        ...

        reveal_type(MyModel.other_again.through)
+       reveal_type(Other.autos.through)

[flaeppe]

I think we need to improve with descriptor types here. e.g.

reveal_type(Other.autos)  # Revealed type is "django.db.models.manager.RelatedManager[myapp.models.MyModel]"
print(type(Other.autos))  # <class 'django.db.models.fields.related_descriptors.ManyToManyDescriptor'>

[superlevure]

I am still getting this error with 4.2.7

[flaeppe]

@superlevure feel free to provide a breaking case and/or open a new issue