[1.11.x] Fixed #27554 -- Fixed prefetch_related() crash when fetching relations in nested Prefetches.

Backport of c0a2b9508a from master

django/db/models/query.py

@@ -1471,6 +1471,11 @@ def prefetch_related_objects(model_instances, *related_lookups):
                 # that we can continue with nullable or reverse relations.
                 new_obj_list = []
                 for obj in obj_list:
+                    if through_attr in getattr(obj, '_prefetched_objects_cache', ()):
+                        # If related objects have been prefetched, use the
+                        # cache rather than the object's through_attr.
+                        new_obj = list(obj._prefetched_objects_cache.get(through_attr))
+                    else:
                         try:
                             new_obj = getattr(obj, through_attr)
                         except exceptions.ObjectDoesNotExist:

docs/releases/1.11.1.txt

@@ -91,3 +91,6 @@ Bugfixes
 
 * Corrected the return type of ``ArrayField(CITextField())`` values retrieved
   from the database (:ticket:`28161`).
+
+* Fixed ``QuerySet.prefetch_related()`` crash when fetching relations in nested
+  ``Prefetch`` objects (:ticket:`27554`).

tests/prefetch_related/tests.py

@@ -1391,3 +1391,37 @@ class Ticket25546Tests(TestCase):
             self.assertListEqual(book1.first_authors[0].happy_place, [self.author1_address1])
             self.assertListEqual(book1.first_authors[1].happy_place, [])
             self.assertListEqual(book2.first_authors[0].happy_place, [self.author2_address1])
+
+
+class ReadPrefetchedObjectsCacheTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.book1 = Book.objects.create(title='Les confessions Volume I')
+        cls.book2 = Book.objects.create(title='Candide')
+        cls.author1 = AuthorWithAge.objects.create(name='Rousseau', first_book=cls.book1, age=70)
+        cls.author2 = AuthorWithAge.objects.create(name='Voltaire', first_book=cls.book2, age=65)
+        cls.book1.authors.add(cls.author1)
+        cls.book2.authors.add(cls.author2)
+        FavoriteAuthors.objects.create(author=cls.author1, likes_author=cls.author2)
+
+    def test_retrieves_results_from_prefetched_objects_cache(self):
+        """
+        When intermediary results are prefetched without a destination
+        attribute, they are saved in the RelatedManager's cache
+        (_prefetched_objects_cache). prefetch_related() uses this cache
+        (#27554).
+        """
+        authors = AuthorWithAge.objects.prefetch_related(
+            Prefetch(
+                'author',
+                queryset=Author.objects.prefetch_related(
+                    # Results are saved in the RelatedManager's cache
+                    # (_prefetched_objects_cache) and do not replace the
+                    # RelatedManager on Author instances (favorite_authors)
+                    Prefetch('favorite_authors__first_book'),
+                ),
+            ),
+        )
+        with self.assertNumQueries(4):
+            # AuthorWithAge -> Author -> FavoriteAuthors, Book
+            self.assertQuerysetEqual(authors, ['<AuthorWithAge: Rousseau>', '<AuthorWithAge: Voltaire>'])