[5.2.x] Fixed #36116 -- Optimized multi-column ForwardManyToOne prefetching.

Rely on ColPairs and TupleIn which support a single column to be specified
to avoid special casing ForwardManyToOne.get_prefetch_querysets().

Thanks Jacob Walls for the report.

Backport of 626d77e52a3f247358514bcf51c761283968099c from main.

1 files changed, 36 insertions, 3 deletions

diff --git a/tests/foreign_object/tests.py b/tests/foreign_object/tests.py
index e288ecd7d4..8b36df29d7 100644
--- a/tests/foreign_object/tests.py
+++ b/tests/foreign_object/tests.py
@@ -4,7 +4,7 @@ import pickle
 from operator import attrgetter
 
 from django.core.exceptions import FieldError
-from django.db import models
+from django.db import connection, models
 from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
 from django.test.utils import isolate_apps
 from django.utils import translation
@@ -247,7 +247,7 @@ class MultiColumnFKTests(TestCase):
         normal_people = [m.person for m in Membership.objects.order_by("pk")]
         self.assertEqual(people, normal_people)
 
-    def test_prefetch_foreignkey_forward_works(self):
+    def test_prefetch_foreignobject_forward(self):
         Membership.objects.create(
             membership_country=self.usa, person=self.bob, group=self.cia
         )
@@ -264,7 +264,40 @@ class MultiColumnFKTests(TestCase):
         normal_people = [m.person for m in Membership.objects.order_by("pk")]
         self.assertEqual(people, normal_people)
 
-    def test_prefetch_foreignkey_reverse_works(self):
+    def test_prefetch_foreignobject_hidden_forward(self):
+        Friendship.objects.create(
+            from_friend_country=self.usa,
+            from_friend_id=self.bob.id,
+            to_friend_country_id=self.usa.id,
+            to_friend_id=self.george.id,
+        )
+        Friendship.objects.create(
+            from_friend_country=self.usa,
+            from_friend_id=self.bob.id,
+            to_friend_country_id=self.soviet_union.id,
+            to_friend_id=self.sam.id,
+        )
+        with self.assertNumQueries(2) as ctx:
+            friendships = list(
+                Friendship.objects.prefetch_related("to_friend").order_by("pk")
+            )
+        prefetch_sql = ctx[-1]["sql"]
+        # Prefetch queryset should be filtered by all foreign related fields
+        # to prevent extra rows from being eagerly fetched.
+        prefetch_where_sql = prefetch_sql.split("WHERE")[-1]
+        for to_field_name in Friendship.to_friend.field.to_fields:
+            to_field = Person._meta.get_field(to_field_name)
+            with self.subTest(to_field=to_field):
+                self.assertIn(
+                    connection.ops.quote_name(to_field.column),
+                    prefetch_where_sql,
+                )
+        self.assertNotIn(" JOIN ", prefetch_sql)
+        with self.assertNumQueries(0):
+            self.assertEqual(friendships[0].to_friend, self.george)
+            self.assertEqual(friendships[1].to_friend, self.sam)
+
+    def test_prefetch_foreignobject_reverse(self):
         Membership.objects.create(
             membership_country=self.usa, person=self.bob, group=self.cia
         )