ManyToManyField.through no longer has _default_manager attribute

[intgr]

Bug report

Following the call for testing in #1740, I tried django-stubs git master and ran into this issue.

For example if MyModel has relation manytomany = models.ManyToManyField(OtherModel)

What's wrong

When accessing MyModel.manytomany.through._default_manager, I'm getting error:

"type[MyModel_manytomany]" has no attribute "_default_manager" [attr-defined]

MyModel.manytomany.through.objects works fine, however.

I can also reproduce this after these changes in django-stubs test suite:

diff --git a/tests/typecheck/fields/test_related.yml b/tests/typecheck/fields/test_related.yml
index 8f21557e..8d0ce289 100644
--- a/tests/typecheck/fields/test_related.yml
+++ b/tests/typecheck/fields/test_related.yml
@@ -337,6 +337,7 @@
         from myapp.models import User
         reveal_type(User().friends)  # N: Revealed type is "django.db.models.fields.related_descriptors.ManyRelatedManager[myapp.models.User]"
         reveal_type(User.friends.through.objects.get())  # N: Revealed type is "myapp.models.User_friends"
+        reveal_type(User.friends.through._default_manager)
         reveal_type(User.friends.through().from_user)  # N: Revealed type is "myapp.models.User"
         reveal_type(User.friends.through().from_user_id)  # N: Revealed type is "builtins.int"
         reveal_type(User.friends.through().to_user)  # N: Revealed type is "myapp.models.User"

E   pytest_mypy_plugins.utils.TypecheckAssertionError: Invalid output: 
E   Actual:
E     ...
E     main:4: error: "Type[User_friends]" has no attribute "_default_manager" (diff)

System information

• OS: macOS 13.6
• python version: 3.11.5
• django version: 4.1.11
• mypy version: 1.5.1
• django-stubs version: (git master, commit 84a085c)
• django-stubs-ext version: 4.2.2

[flaeppe]

Aha, nice find! _default_manager is a forgotten symbol:

django-stubs/mypy_django_plugin/transformers/models.py

Lines 683 to 742 in 84a085c

	through_model = self.add_new_class_for_current_module(
	through_model_name, bases=[Instance(model_base, [])]
	)
	# We attempt to be a bit clever here and store the generated through model's fullname in
	# the metadata of the class containing the 'ManyToManyField' call expression, where its
	# identifier is the field name of the 'ManyToManyField'. This would allow the containing
	# model to always find the implicit through model, so that it doesn't get lost.
	model_metadata = helpers.get_django_metadata(self.model_classdef.info)
	model_metadata.setdefault("m2m_throughs", {})
	model_metadata["m2m_throughs"][m2m_field_name] = through_model.fullname
	# Add a 'pk' symbol to the model class
	helpers.add_new_sym_for_info(
	through_model, name="pk", sym_type=self.default_pk_instance.copy_modified()
	)
	# Add an 'id' symbol to the model class
	helpers.add_new_sym_for_info(
	through_model, name="id", sym_type=self.default_pk_instance.copy_modified()
	)
	# Add the foreign key to the model containing the 'ManyToManyField' call:
	# <containing_model> or from_<model>
	from_name = (
	f"from_{self.model_classdef.name.lower()}" if args.to.self else self.model_classdef.name.lower()
	)
	helpers.add_new_sym_for_info(
	through_model,
	name=from_name,
	sym_type=Instance(
	fk_field,
	[
	helpers.convert_any_to_type(fk_set_type, Instance(self.model_classdef.info, [])),
	helpers.convert_any_to_type(fk_get_type, Instance(self.model_classdef.info, [])),
	],
	),
	)
	# Add the foreign key's '_id' field: <containing_model>_id or from_<model>_id
	helpers.add_new_sym_for_info(through_model, name=f"{from_name}_id", sym_type=from_pk.copy_modified())
	# Add the foreign key to the model on the opposite side of the relation
	# i.e. the model given as 'to' argument to the 'ManyToManyField' call:
	# <other_model> or to_<model>
	to_name = f"to_{args.to.model.type.name.lower()}" if args.to.self else args.to.model.type.name.lower()
	helpers.add_new_sym_for_info(
	through_model,
	name=to_name,
	sym_type=Instance(
	fk_field,
	[
	helpers.convert_any_to_type(fk_set_type, args.to.model),
	helpers.convert_any_to_type(fk_get_type, args.to.model),
	],
	),
	)
	# Add the foreign key's '_id' field: <other_model>_id or to_<model>_id
	other_pk = self.get_pk_instance(args.to.model.type)
	helpers.add_new_sym_for_info(through_model, name=f"{to_name}_id", sym_type=other_pk.copy_modified())
	# Add a manager named 'objects'
	helpers.add_new_sym_for_info(
	through_model,
	name="objects",
	sym_type=Instance(manager_info, [Instance(through_model, [])]),
	)