Prefetch related is not working when used GFK for model that uses UUID field as PK.

[Timur]

How to reproduce:

• create model with UUID as primary key

class Foo(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    ...

• create another model with GFK to model Foo

class Bar(models.Model):
    foo_content_type = models.ForeignKey(
        ContentType, related_name='actor',
        on_delete=models.CASCADE, db_index=True
    )
    foo_object_id = models.CharField(max_length=255, db_index=True)
    foo = GenericForeignKey('foo_content_type', 'foo_object_id')
    ...

• and try to get queryset with prefetch related (django orm engine return None for attribute foo):

Bar.objects.all().prefetch_related('foo')

Thanks a lot for your attention! Also i wanna point out some related bug report from third party library in which previously i faced with that issue, maybe it would useful – ​https://github.com/justquick/django-activity-stream/issues/245

[Mariusz Felisiak]

Thanks for the report, however GenericForeignKey doesn't support prefetch_related(), from the same reason it doesn't not support also filter(), exclude(), etc. Please see ​documentation:

Due to the way GenericForeignKey is implemented, you cannot use such fields directly with filters (filter() and exclude(), for example) via the database API. Because a GenericForeignKey isn’t a normal field object, these examples will not work:
...

Provided example works as expected with UUID PK, e.g.:

>>> for bar in Bar.objects.all():
...    print(bar, bar.foo)
Bar object (4) Foo object (e6bdfd5e-51a8-49c7-9a7a-998452fad6aa)

[Mariusz Felisiak]

Sorry for previous comment.

This should work. Currently it works when foo_object_id is an UUIDField but doesn't work when it is a CharField.

[Mariusz Felisiak]

IMO, adding get_prep_value() to the UUIDField should fix this issue 🤔:

django/db/models/fields/__init__.py

@@ -2335 +2335 @@
             return value
         return value.hex
 
+    def get_prep_value(self, value):
+        value = super().get_prep_value(value)
+        if value is None:
+            return None
+        return self.to_python(value)
+
     def to_python(self, value):
         if value is not None and not isinstance(value, uuid.UUID):
             input_form = 'int' if isinstance(value, int) else 'hex'

[Simon Charette]

Yeah that seems like the correct approach. I was about to suggest

django/contrib/contenttypes/fields.py

@@ -202 +202 @@
             else:
                 model = self.get_content_type(id=ct_id,
                                               using=obj._state.db).model_class()
-                return (model._meta.pk.get_prep_value(getattr(obj, self.fk_field)),
+                return (model._meta.pk.to_python(getattr(obj, self.fk_field)),
                         model)

But it looks like this is really just an issue with UUIDField.

[Mariusz Felisiak]

​PR

[Timur]

Replying to felixxm: Thanks for your work!

​PR

[GitHub <noreply@…>]

In 1afbc96:

Fixed #30343 -- Fixed prefetch_related() for GenericForeignKey when PK of related field is UUIDField.