prefetch_related_objects fails to cache UUID FKs when the string representation of a UUID is used

[Selcuk Ayguney]

Consider the following models:

class Pet(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    name = models.CharField(max_length=20)


class Toy(models.Model):
    pet = models.ForeignKey(Pet, models.CASCADE, related_name="toys")
    name = models.CharField(max_length=20)

The following works and caches the deferred pet instance as expected:

pet = Pet.objects.create(name="Fifi")
toy_bone = Toy(pet_id=str(pet.id), name="Bone")
print(toy_bone.pet)

It fails if prefetch_related_objects is used:

pet = Pet.objects.create(name="Fifi")
toy_bone = Toy(pet_id=str(pet.id), name="Bone")
prefetch_related_objects([toy_bone], "pet")
print(toy_bone.pet)

    raise self.RelatedObjectDoesNotExist(
    ^^^^^^^^^^^^^^^^^
prefetch_related.models.Toy.pet.RelatedObjectDoesNotExist: Toy has no pet.

The behaviour is inconsistent as str to UUID conversion is automatically performed in the first example.

One may argue that the use of prefetch_related_objects is redundant in this case. However, it is useful in async code where deferred lookups are not automatic.

Unit test to reproduce the issue and the suggested fix:
​https://github.com/django/django/pull/18132