[5.2.x] Fixed #36135 -- Fixed reverse GenericRelation prefetching.

The get_(local|foreign)_related_value methods of GenericRelation must be
reversed because it defines (from|to)_fields and associated related_fields
in the reversed order as it's effectively a reverse GenericForeignKey
itself.

The related value methods must also account for the fact that referenced
primary key values might be stored as a string on the model defining the
GenericForeignKey but as integer on the model defining the GenericRelation.
This is achieved by calling the to_python method of the involved content type
in get_foreign_related_value just like GenericRelatedObjectManager does.

Lastly reverse many-to-one manager's prefetch_related_querysets should use
set_cached_value instead of direct attribute assignment as direct assignment
might are disallowed on ReverseManyToOneDescriptor descriptors. This is likely
something that was missed in f5233dc (refs #32511) when the is_cached guard
was added.

Thanks 1xinghuan for the report.

Backport of 198b30168d4e94af42e0dc7967bd3259b5c5790b from main.

django/contrib/contenttypes/fields.py

@@ -402,6 +402,20 @@ class GenericRelation(ForeignObject):
             )
         ]
 
+    def get_local_related_value(self, instance):
+        return self.get_instance_value_for_fields(instance, self.foreign_related_fields)
+
+    def get_foreign_related_value(self, instance):
+        # We (possibly) need to convert object IDs to the type of the
+        # instances' PK in order to match up instances during prefetching.
+        return tuple(
+            foreign_field.to_python(val)
+            for foreign_field, val in zip(
+                self.foreign_related_fields,
+                self.get_instance_value_for_fields(instance, self.local_related_fields),
+            )
+        )
+
     def _get_path_info_with_parent(self, filtered_relation):
         """
         Return the path that joins the current model through any parent models.

django/db/models/fields/related_descriptors.py

@@ -794,7 +794,7 @@ def create_reverse_many_to_one_manager(superclass, rel):
             for rel_obj in queryset:
                 if not self.field.is_cached(rel_obj):
                     instance = instances_dict[rel_obj_attr(rel_obj)]
-                    setattr(rel_obj, self.field.name, instance)
+                    self.field.set_cached_value(rel_obj, instance)
             cache_name = self.field.remote_field.cache_name
             return queryset, rel_obj_attr, instance_attr, False, cache_name, False
 

tests/prefetch_related/tests.py

@@ -1257,6 +1257,20 @@ class GenericRelationTests(TestCase):
                 ],
             )
 
+    def test_reverse_generic_relation(self):
+        # Create two distinct bookmarks to ensure the bookmark and
+        # tagged item models primary are offset.
+        first_bookmark = Bookmark.objects.create()
+        second_bookmark = Bookmark.objects.create()
+        TaggedItem.objects.create(
+            content_object=first_bookmark, favorite=second_bookmark
+        )
+        with self.assertNumQueries(2):
+            obj = TaggedItem.objects.prefetch_related("favorite_bookmarks").get()
+        with self.assertNumQueries(0):
+            prefetched_bookmarks = obj.favorite_bookmarks.all()
+            self.assertQuerySetEqual(prefetched_bookmarks, [second_bookmark])
+
 
 class MultiTableInheritanceTest(TestCase):
     @classmethod