5.2.3 regression: Prefetch with a related queryset produces arg-type errors with django-money

[stianjensen]

Bug report

What's wrong

Using Prefetch with a related queryset now produces arg-type errors after upgrading from django-stubs==5.2.2 to 5.2.5.

This happens only when the base model includes a MoneyField from django-money. Without MoneyField, the same code type-checks fine.

Minimal example:

from django.db import models
from django.db.models import Prefetch
from djmoney.models.fields import MoneyField

class Author(models.Model):
    name = models.CharField(max_length=100)
    # Trigger: the presence of a MoneyField
    advance = MoneyField(max_digits=10, decimal_places=2, default_currency="USD")

class Book(models.Model):
    author = models.ForeignKey(Author, related_name="books", on_delete=models.CASCADE)
    title = models.CharField(max_length=100)

# ❌ Fails with django-stubs 5.2.5
qs = Author.objects.prefetch_related(
    Prefetch(
        "books",
        queryset=Book.objects.select_related("author"),
    )
)

Running mypy on this with django-stubs==5.2.5 yields:

error: Argument 1 to "prefetch_related" of "QuerySet" has incompatible type "Prefetch[Literal['books'], _PrefetchedQuerySetT, _ToAttrT]"; expected "str | Prefetch[_LookupT, _PrefetchedQuerySetT, _ToAttrT]"  [arg-type]
error: Argument 1 to "Prefetch" has incompatible type "str"; expected "_LookupT"  [arg-type]
error: Argument "queryset" to "Prefetch" has incompatible type "QuerySet[Book, Book]"; expected "_PrefetchedQuerySetT | None"  [arg-type]

How is that should be

The code should still type-check successfully, since I believe this is valid Django usage, and it worked in previous versions.

I'm not sure exactly what django-money is doing to trigger this here, but unless that pattern is not supposed to be supported, I think it would be good for django-stubs to still support this. Otherwise I'll try to investigate whether django-money's model wrapping with a custom manager needs a rework.

System information

• OS: MacOS 15.6.1
• python version: 3.13.2
• django version: 5.2.6
• mypy version: 1.18.2
• django-stubs version: 5.2.5
• django-stubs-ext version: 5.2.5
• django-money version: 3.5.4

[stianjensen]

For reference, here is the implementation of the automatic manager wrapping that django-money performs:
https://github.com/django-money/django-money/blob/57f6d8ccb8a89476379656b1b7333e8195d2d767/djmoney/models/managers.py#L226

And here it's applied to models with a MoneyField:
https://github.com/django-money/django-money/blob/57f6d8ccb8a89476379656b1b7333e8195d2d767/djmoney/models/fields.py#L366

[UnknownPlatypus]

For reference, here is the implementation of the automatic manager wrapping that django-money performs:

I'll try to see if we can make this work but the dark magic happening in https://github.com/django-money/django-money/blob/57f6d8ccb8a89476379656b1b7333e8195d2d767/djmoney/models/managers.py#L226 is probably hard to understand for the plugin, especially since these functions seem to be untyped ?

[stianjensen]

If adding proper stubs to the manager wrapping helps, I can look into that.

[UnknownPlatypus]

I've tried to remove the setup_managers thingy in django money and this fixes it. I even get actual errors:

        qs = Author.objects.prefetch_related(
            Prefetch(
                "AAA", # E: Cannot find "AAA" on "Author" object, "AAA" is an invalid parameter to "prefetch_related()"  [misc]
                queryset=Book.objects.select_related("author"),
            )
        )

This issue is that the objects manager is not explicitly annotated (I believe adding objects: models.Manager might solve your issue btw) so the mypy plugins has to try to resolve the type here:

django-stubs/mypy_django_plugin/transformers/models.py

Line 379 in 736e30f

incomplete_manager_defs = set()

But it gets a weird, dynamically generated 'djmoney.models.managers.money_manager.<locals>.MoneyManager' that cannot be resolved to a Manager.

I think money_manager need to find a way to make the monkeypatch approach to work, to automatically decorate get_queryset with add_money_comprehension_to_queryset without requiring a dynamic MoneyManager type.

Overall the whole patching approach seems quite brittle, and a bit scary. This will be difficult to represent well in the typing system if the approach stays the same

Test case to reproduce (require to pip install django-money)

# Regression test for #2842
-   case: prefetch_related_django_money_patched_manager
    main: |
        from myapp.models import Author, Book
        from django.db.models import Prefetch


        qs = Author.objects.prefetch_related(
            Prefetch(
                "books",
                queryset=Book.objects.select_related("author"),
            )
        )

    installed_apps:
        - myapp
    files:
        -   path: myapp/__init__.py
        -   path: myapp/models.py
            content: |
                from django.db import models
                from django.db.models import Prefetch
                from djmoney.models.fields import MoneyField

                class Author(models.Model):
                    name = models.CharField(max_length=100)
                    # Trigger: the presence of a MoneyField
                    advance = MoneyField(max_digits=10, decimal_places=2, default_currency="USD")

                    objects: models.Manager
                class Book(models.Model):
                    author = models.ForeignKey(Author, related_name="books", on_delete=models.CASCADE)
                    title = models.CharField(max_length=100)