Fixed #29908 -- Fixed setting of foreign key after related set access if ForeignKey uses to_field.

Adjusted known related objects handling of target fields which relies on
from and to_fields and has the side effect of fixing a bug bug causing
N+1 queries when using reverse foreign objects.

Thanks Carsten Fuchs for the report.

3 files changed, 17 insertions, 9 deletions

diff --git a/tests/foreign_object/tests.py b/tests/foreign_object/tests.py
index 59d4357802..6af6def6d7 100644
--- a/tests/foreign_object/tests.py
+++ b/tests/foreign_object/tests.py
@@ -69,12 +69,10 @@ class MultiColumnFKTests(TestCase):
             membership_country_id=self.soviet_union.id, person_id=self.bob.id,
             group_id=self.republican.id)
 
-        self.assertQuerysetEqual(
-            self.bob.membership_set.all(), [
-                self.cia.id
-            ],
-            attrgetter("group_id")
-        )
+        with self.assertNumQueries(1):
+            membership = self.bob.membership_set.get()
+            self.assertEqual(membership.group_id, self.cia.id)
+            self.assertIs(membership.person, self.bob)
 
     def test_query_filters_correctly(self):
 
@@ -198,8 +196,11 @@ class MultiColumnFKTests(TestCase):
                 list(p.membership_set.all())
                 for p in Person.objects.prefetch_related('membership_set').order_by('pk')]
 
-        normal_membership_sets = [list(p.membership_set.all())
-                                  for p in Person.objects.order_by('pk')]
+        with self.assertNumQueries(7):
+            normal_membership_sets = [
+                list(p.membership_set.all())
+                for p in Person.objects.order_by('pk')
+            ]
         self.assertEqual(membership_sets, normal_membership_sets)
 
     def test_m2m_through_forward_returns_valid_members(self):
diff --git a/tests/many_to_one/models.py b/tests/many_to_one/models.py
index 2f98bebdc0..ef784cfbe2 100644
--- a/tests/many_to_one/models.py
+++ b/tests/many_to_one/models.py
@@ -71,7 +71,7 @@ class Child(models.Model):
 
 
 class ToFieldChild(models.Model):
-    parent = models.ForeignKey(Parent, models.CASCADE, to_field='name')
+    parent = models.ForeignKey(Parent, models.CASCADE, to_field='name', related_name='to_field_children')
 
 
 # Multiple paths to the same model (#7110, #7125)
diff --git a/tests/many_to_one/tests.py b/tests/many_to_one/tests.py
index b04e6ad77a..28430256dc 100644
--- a/tests/many_to_one/tests.py
+++ b/tests/many_to_one/tests.py
@@ -672,3 +672,10 @@ class ManyToOneTests(TestCase):
         child = ToFieldChild.objects.create(parent=parent)
         with self.assertNumQueries(0):
             self.assertIs(child.parent, parent)
+
+    def test_reverse_foreign_key_instance_to_field_caching(self):
+        parent = Parent.objects.create(name='a')
+        ToFieldChild.objects.create(parent=parent)
+        child = parent.to_field_children.get()
+        with self.assertNumQueries(0):
+            self.assertIs(child.parent, parent)