From 036f160733fcceb10975bf03c2fc9ffd9c114612 Mon Sep 17 00:00:00 2001
From: Bob Renwick <bob.renwick@gmail.com>
Date: Tue, 6 Oct 2020 10:04:11 +0000
Subject: [PATCH] Refs #20577 -- Deferred filtering of prefetched related
 querysets by reverse m2o relation.

---
 .../db/models/fields/related_descriptors.py   |  1 +
 tests/prefetch_related/tests.py               | 21 +++++++++++--------
 2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/django/db/models/fields/related_descriptors.py b/django/db/models/fields/related_descriptors.py
index 7ade10ca6e9..872a4c98dc6 100644
--- a/django/db/models/fields/related_descriptors.py
+++ b/django/db/models/fields/related_descriptors.py
@@ -581,6 +581,7 @@ def create_reverse_many_to_one_manager(superclass, rel):
             queryset._add_hints(instance=self.instance)
             if self._db:
                 queryset = queryset.using(self._db)
+            queryset._defer_next_filter = True
             queryset = queryset.filter(**self.core_filters)
             for field in self.field.foreign_related_fields:
                 val = getattr(self.instance, field.attname)
diff --git a/tests/prefetch_related/tests.py b/tests/prefetch_related/tests.py
index 740e9cb1b58..c368e040eaf 100644
--- a/tests/prefetch_related/tests.py
+++ b/tests/prefetch_related/tests.py
@@ -294,17 +294,20 @@ class PrefetchRelatedTests(TestDataMixin, TestCase):
 
     def test_filter_deferred(self):
         """
-        Related filtering of prefetched querysets is deferred until necessary.
+        Related filtering of prefetched querysets is deferred on m2m and
+        reverse m2o relations until necessary.
         """
         add_q = Query.add_q
-        with mock.patch.object(
-            Query,
-            'add_q',
-            autospec=True,
-            side_effect=lambda self, q: add_q(self, q),
-        ) as add_q_mock:
-            list(Book.objects.prefetch_related('authors'))
-            self.assertEqual(add_q_mock.call_count, 1)
+        for relation in ['authors', 'first_time_authors']:
+            with self.subTest(relation=relation):
+                with mock.patch.object(
+                    Query,
+                    'add_q',
+                    autospec=True,
+                    side_effect=lambda self, q: add_q(self, q),
+                ) as add_q_mock:
+                    list(Book.objects.prefetch_related(relation))
+                    self.assertEqual(add_q_mock.call_count, 1)
 
 
 class RawQuerySetTests(TestDataMixin, TestCase):