Django - select_related() ignores annotations set by prefetch_related()

select_related_ignores_annotations.py

# AnnotatedModel has `is_valid` automatically annotated
# by the custom manager which is set as the default manager

instance = AnnotatedModel.objects.get(pk=1)
assert instance.is_valid is True # always set to True/False

# DifferentModel has FK to AnnotatedModel and
# the manager uses Prefetch to make sure all the
# annotations are there when related object is accessed

class DifferentModelManager(models.Manager):
    def get_queryset(self):
        pf = Prefetch(
            'annotatedmodel', queryset=AnnotatedModel.objects.all()
        )
        return super().get_queryset().prefetch_related(pf)


class DifferentModel(models.Model):
    objects = DifferentModelManager()

dm_instance = DifferentModel.objects.get(pk=1)
assert dm_instance.annotatedmodel.is_valid is True

# the error happens when select_related() is used
dm_instance = DifferentModel.objects.select_related('annotatedmodel__othermodel').get(pk=1)
assert dm_instance.annotatedmodel.is_valid is True

> E AttributeError: 'AnnotatedModel' object has no attribute 'is_valid'