[4.0.x] Fixed #33008 -- Fixed prefetch_related() for deleted GenericForeignKeys.

Thanks Simon Charette for the implementation idea.

Backport of cc4cb95bef from main

django/contrib/contenttypes/fields.py

@@ -213,7 +213,7 @@ class GenericForeignKey(FieldCacheMixin):
             gfk_key,
             True,
             self.name,
-            True,
+            False,
         )
 
     def __get__(self, instance, cls=None):
@@ -229,6 +229,8 @@ class GenericForeignKey(FieldCacheMixin):
         pk_val = getattr(instance, self.fk_field)
 
         rel_obj = self.get_cached_value(instance, default=None)
+        if rel_obj is None and self.is_cached(instance):
+            return rel_obj
         if rel_obj is not None:
             ct_match = ct_id == self.get_content_type(obj=rel_obj, using=instance._state.db).id
             pk_match = rel_obj._meta.pk.to_python(pk_val) == rel_obj.pk

tests/contenttypes_tests/test_fields.py

@@ -2,14 +2,14 @@ import json
 
 from django.contrib.contenttypes.fields import GenericForeignKey
 from django.db import models
-from django.test import SimpleTestCase, TestCase
+from django.test import TestCase
 from django.test.utils import isolate_apps
 
-from .models import Answer, Question
+from .models import Answer, Post, Question
 
 
 @isolate_apps('contenttypes_tests')
-class GenericForeignKeyTests(SimpleTestCase):
+class GenericForeignKeyTests(TestCase):
 
     def test_str(self):
         class Model(models.Model):
@@ -24,6 +24,19 @@ class GenericForeignKeyTests(SimpleTestCase):
         with self.assertRaisesMessage(ValueError, "Custom queryset can't be used for this lookup."):
             Answer.question.get_prefetch_queryset(Answer.objects.all(), Answer.objects.all())
 
+    def test_get_object_cache_respects_deleted_objects(self):
+        question = Question.objects.create(text='Who?')
+        post = Post.objects.create(title='Answer', parent=question)
+
+        question_pk = question.pk
+        Question.objects.all().delete()
+
+        post = Post.objects.get(pk=post.pk)
+        with self.assertNumQueries(1):
+            self.assertEqual(post.object_id, question_pk)
+            self.assertIsNone(post.parent)
+            self.assertIsNone(post.parent)
+
 
 class GenericRelationTests(TestCase):
 

tests/prefetch_related/tests.py

@@ -1033,6 +1033,24 @@ class GenericRelationTests(TestCase):
         # instance returned by the manager.
         self.assertEqual(list(bookmark.tags.all()), list(bookmark.tags.all().all()))
 
+    def test_deleted_GFK(self):
+        TaggedItem.objects.create(tag='awesome', content_object=self.book1)
+        TaggedItem.objects.create(tag='awesome', content_object=self.book2)
+        ct = ContentType.objects.get_for_model(Book)
+
+        book1_pk = self.book1.pk
+        self.book1.delete()
+
+        with self.assertNumQueries(2):
+            qs = TaggedItem.objects.filter(tag='awesome').prefetch_related('content_object')
+            result = [
+                (tag.object_id, tag.content_type_id, tag.content_object) for tag in qs
+            ]
+            self.assertEqual(result, [
+                (book1_pk, ct.pk, None),
+                (self.book2.pk, ct.pk, self.book2),
+            ])
+
 
 class MultiTableInheritanceTest(TestCase):