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.

1 files changed, 14 insertions, 0 deletions

diff --git a/django/contrib/contenttypes/fields.py b/django/contrib/contenttypes/fields.py
index 0e68c2eef0..272cb2b02c 100644
--- a/django/contrib/contenttypes/fields.py
+++ b/django/contrib/contenttypes/fields.py
@@ -389,6 +389,20 @@ 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.