Fixed #34819 -- Made GenericForeignKey prefetching use matching pk representations.

Ensured that rel_obj_attr and instance_attr return matching (pk, cls) tuples
in GenericForeignKey.get_prefetch_queryset(), preventing mismatches when
prefetching related objects where pk and get_prep_value() differ. Using
value_to_string() also makes this code compatible with composite primary keys.

3 files changed, 20 insertions, 5 deletions

diff --git a/django/contrib/contenttypes/fields.py b/django/contrib/contenttypes/fields.py
index 272cb2b02c..f9d896bc13 100644
--- a/django/contrib/contenttypes/fields.py
+++ b/django/contrib/contenttypes/fields.py
@@ -205,14 +205,11 @@ class GenericForeignKey(FieldCacheMixin, Field):
                 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)),
-                    model,
-                )
+                return str(getattr(obj, self.fk_field)), model
 
         return (
             ret_val,
-            lambda obj: (obj.pk, obj.__class__),
+            lambda obj: (obj._meta.pk.value_to_string(obj), obj.__class__),
             gfk_key,
             True,
             self.name,
diff --git a/tests/prefetch_related/models.py b/tests/prefetch_related/models.py
index 2f37cde1c8..e12eabdfb2 100644
--- a/tests/prefetch_related/models.py
+++ b/tests/prefetch_related/models.py
@@ -216,6 +216,15 @@ class Comment(models.Model):
         ordering = ["id"]
 
 
+class ArticleCustomUUID(models.Model):
+    class CustomUUIDField(models.UUIDField):
+        def get_prep_value(self, value):
+            return str(value)
+
+    id = CustomUUIDField(primary_key=True, default=uuid.uuid4)
+    name = models.CharField(max_length=30)
+
+
 # Models for lookup ordering tests
 
 
diff --git a/tests/prefetch_related/tests.py b/tests/prefetch_related/tests.py
index 72c499df7f..49acfdd7c8 100644
--- a/tests/prefetch_related/tests.py
+++ b/tests/prefetch_related/tests.py
@@ -16,6 +16,7 @@ from django.test.utils import CaptureQueriesContext
 
 from .models import (
     Article,
+    ArticleCustomUUID,
     Author,
     Author2,
     AuthorAddress,
@@ -1179,6 +1180,14 @@ class GenericRelationTests(TestCase):
         qs = Comment.objects.prefetch_related("content_object_uuid")
         self.assertEqual([c.content_object_uuid for c in qs], [article])
 
+    def test_prefetch_GFK_uses_prepped_primary_key(self):
+        article = ArticleCustomUUID.objects.create(name="Blanche")
+        Comment.objects.create(comment="Enchantment", content_object_uuid=article)
+        obj = Comment.objects.prefetch_related("content_object_uuid").get(
+            comment="Enchantment"
+        )
+        self.assertEqual(obj.content_object_uuid, article)
+
     def test_prefetch_GFK_fk_pk(self):
         book = Book.objects.create(title="Poems")
         book_with_year = BookWithYear.objects.create(book=book, published_year=2019)