Fixed #30493 -- Fixed prefetch_related() for GenericRelation with different content types.

django/contrib/contenttypes/fields.py

@@ -1,3 +1,6 @@
+import functools
+import itertools
+import operator
 from collections import defaultdict
 
 from django.contrib.contenttypes.models import ContentType
@@ -515,17 +518,18 @@ def __init__(self, instance=None):
             self.instance = instance
 
             self.model = rel.model
-
-            content_type = ContentType.objects.db_manager(instance._state.db).get_for_model(
-                instance, for_concrete_model=rel.field.for_concrete_model)
-            self.content_type = content_type
+            self.get_content_type = functools.partial(
+                ContentType.objects.db_manager(instance._state.db).get_for_model,
+                for_concrete_model=rel.field.for_concrete_model,
+            )
+            self.content_type = self.get_content_type(instance)
             self.content_type_field_name = rel.field.content_type_field_name
             self.object_id_field_name = rel.field.object_id_field_name
             self.prefetch_cache_name = rel.field.attname
             self.pk_val = instance.pk
 
             self.core_filters = {
-                '%s__pk' % self.content_type_field_name: content_type.id,
+                '%s__pk' % self.content_type_field_name: self.content_type.id,
                 self.object_id_field_name: self.pk_val,
             }
 
@@ -564,19 +568,28 @@ def get_prefetch_queryset(self, instances, queryset=None):
 
             queryset._add_hints(instance=instances[0])
             queryset = queryset.using(queryset._db or self._db)
-
-            query = {
-                '%s__pk' % self.content_type_field_name: self.content_type.id,
-                '%s__in' % self.object_id_field_name: {obj.pk for obj in instances}
-            }
-
+            # Group instances by content types.
+            content_type_queries = (
+                models.Q(**{
+                    '%s__pk' % self.content_type_field_name: content_type_id,
+                    '%s__in' % self.object_id_field_name: {obj.pk for obj in objs}
+                })
+                for content_type_id, objs in itertools.groupby(
+                    sorted(instances, key=lambda obj: self.get_content_type(obj).pk),
+                    lambda obj: self.get_content_type(obj).pk,
+                )
+            )
+            query = functools.reduce(operator.or_, content_type_queries)
             # We (possibly) need to convert object IDs to the type of the
             # instances' PK in order to match up instances:
             object_id_converter = instances[0]._meta.pk.to_python
             return (
-                queryset.filter(**query),
-                lambda relobj: object_id_converter(getattr(relobj, self.object_id_field_name)),
-                lambda obj: obj.pk,
+                queryset.filter(query),
+                lambda relobj: (
+                    object_id_converter(getattr(relobj, self.object_id_field_name)),
+                    relobj.content_type_id
+                ),
+                lambda obj: (obj.pk, self.get_content_type(obj).pk),
                 False,
                 self.prefetch_cache_name,
                 False,

tests/generic_relations/tests.py

@@ -546,6 +546,24 @@ def test_add_then_remove_after_prefetch(self):
         platypus.tags.remove(weird_tag)
         self.assertSequenceEqual(platypus.tags.all(), [furry_tag])
 
+    def test_prefetch_related_different_content_types(self):
+        TaggedItem.objects.create(content_object=self.platypus, tag='prefetch_tag_1')
+        TaggedItem.objects.create(
+            content_object=Vegetable.objects.create(name='Broccoli'),
+            tag='prefetch_tag_2',
+        )
+        TaggedItem.objects.create(
+            content_object=Animal.objects.create(common_name='Bear'),
+            tag='prefetch_tag_3',
+        )
+        qs = TaggedItem.objects.filter(
+            tag__startswith='prefetch_tag_',
+        ).prefetch_related('content_object', 'content_object__tags')
+        with self.assertNumQueries(4):
+            tags = list(qs)
+        for tag in tags:
+            self.assertSequenceEqual(tag.content_object.tags.all(), [tag])
+
 
 class ProxyRelatedModelTest(TestCase):
     def test_default_behavior(self):