Improve ManyToManyDescriptor and fix Model.<manytomany>.through typing

[flaeppe]

ManyToManyDescriptor is now extended to take 1 new type argument, which is the target model/other side of the relation.

The plugin is updated to:

• Set a ManyToManyDescriptor instance instead of a related manager, for reverse relations of a ManyToManyField
• Produce a manager class with ManyRelatedManager and a model's default manager as bases for both sides of a many-to-many relation

Related issues

Closes #1800

[sobolevn]

Generally LGTM, just one idea: maybe we should add at least one test that checks an error from this new guarantee that we added?

[flaeppe]

Just realised that the plugin needs to make the returned ManyRelatedManager here more accurate, so that it's possible to find custom queryset/manager methods.

Quite surprised we're missing tests for that actually.

It probably would've been easiest if ManyRelatedManager was another type argument, but I don't think it can. Since it's not possible to pass any dynamically generated managers as type arguments (ref #1699).

[flaeppe]

I've adjusted so that we imitate Django's behaviour by having the plugin create a manager class with ManyRelatedManager and the _To model's default manager as bases.

Ref: https://github.com/django/django/blob/46df3ab244e1688bd186f0bfbfea6a354097a910/django/db/models/fields/related_descriptors.py#L1032

[flaeppe]

Generally LGTM, just one idea: maybe we should add at least one test that checks an error from this new guarantee that we added?

Some parts changed since you looked at it. But I've still only added "happy"/inclusive asserts, not sure what asserts on errors we'd like to add? I find it a bit hard since it kind of appears as having to test "everything else". e.g. accessing attributes unknown for ManyToManyDescriptor, but then I'd say we're testing attribute access in mypy.

[intgr]

Btw this needs merge/rebase

@sobolevn Do you have time to re-review this?

[flaeppe]

I've rebased and resolved conflicts just now.

[intgr]

Looks good to me, only had some nitpick comments.

[flaeppe]

Thanks for the review. I'll look in to the needed fixes as soon as I'm able to squeeze in some time

[intgr]

Thanks, looks good. I like how you refactored get_related_manager_and_model().

[intgr]

This issue is likely related:

• AssertionError: Cannot find component <model>_ManyRelatedManager for <app>.models.<model>_ManyRelatedManager #1861