id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
37267	Prefetch can populate a reverse foreign key cache with instances of an unrelated queryset model	beingfaisal		"`Prefetch()` accepts a custom queryset whose model is unrelated to the
model represented by a reverse foreign key lookup. If both models contain
a foreign key with the same name, Django can populate the relationship
cache with instances of the wrong model.

For example:

{{{#!python
class Patient(models.Model):
    pass


class ChatAlert(models.Model):
    patient = models.ForeignKey(
        Patient,
        related_name=""chatalerts"",
        on_delete=models.CASCADE,
    )


class HealthAlert(models.Model):
    patient = models.ForeignKey(
        Patient,
        related_name=""healthalerts"",
        on_delete=models.CASCADE,
    )
}}}

The following prefetch is accepted:

{{{#!python
patient = Patient.objects.create()
health_alert = HealthAlert.objects.create(patient=patient)

patient = Patient.objects.prefetch_related(
    Prefetch(
        ""chatalerts"",
        queryset=HealthAlert.objects.all(),
    )
).get(pk=patient.pk)

alerts = list(patient.chatalerts.all())
}}}

`Patient.chatalerts` represents the reverse side of
`ChatAlert.patient`, so it should only contain `ChatAlert` instances.
Instead, `alerts` contains the `HealthAlert` instance.

Both models expose `patient` and `patient_id`, so the reverse foreign key
prefetch machinery can filter and group the incompatible queryset without
raising an error. With a different schema, the same invalid configuration
may instead fail later with a less clear field or attribute error.

I expect evaluating the prefetch to raise `ValueError` when the custom
queryset model is incompatible with the relationship model.

A regression test using existing `prefetch_related` test models fails with:

{{{
AssertionError: ValueError not raised
}}}

Custom querysets using subclasses of the expected relationship model must
remain supported, as established by #36432. Therefore, compatibility should
be directional: the supplied queryset model may be the expected model or
one of its subclasses, but not an unrelated model.

AI assistance: OpenAI Codex (GPT-5) helped analyze and draft this report.
I manually verified the reproduction."	Bug	new	Database layer (models, ORM)	dev	Normal		Prefetch prefetch_related queryset model	beingfaisal	Unreviewed	0	0	0	0	0	0