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.

django/db/models/fields/related_descriptors.py

@@ -576,7 +576,16 @@ def create_reverse_many_to_one_manager(superclass, rel):
                 val = getattr(self.instance, field.attname)
                 if val is None or (val == '' and empty_strings_as_null):
                     return queryset.none()
-            queryset._known_related_objects = {self.field: {self.instance.pk: self.instance}}
+            if self.field.many_to_one:
+                # Guard against field-like objects such as GenericRelation
+                # that abuse create_reverse_many_to_one_manager() with reverse
+                # one-to-many relationships instead and break known related
+                # objects assignment.
+                rel_obj_id = tuple([
+                    getattr(self.instance, target_field.attname)
+                    for target_field in self.field.get_path_info()[-1].target_fields
+                ])
+                queryset._known_related_objects = {self.field: {rel_obj_id: self.instance}}
             return queryset
 
         def _remove_prefetched_objects(self):

django/db/models/query.py

@@ -61,6 +61,16 @@ class ModelIterable(BaseIterable):
         init_list = [f[0].target.attname
                      for f in select[model_fields_start:model_fields_end]]
         related_populators = get_related_populators(klass_info, select, db)
+        known_related_objects = [
+            (field, related_objs, [
+                operator.attrgetter(
+                    field.attname
+                    if from_field == 'self' else
+                    queryset.model._meta.get_field(from_field).attname
+                )
+                for from_field in field.from_fields
+            ]) for field, related_objs in queryset._known_related_objects.items()
+        ]
         for row in compiler.results_iter(results):
             obj = model_cls.from_db(db, init_list, row[model_fields_start:model_fields_end])
             for rel_populator in related_populators:
@@ -69,17 +79,16 @@ class ModelIterable(BaseIterable):
                 for attr_name, col_pos in annotation_col_map.items():
                     setattr(obj, attr_name, row[col_pos])
 
-            # Add the known related objects to the model, if there are any
-            if queryset._known_related_objects:
-                for field, rel_objs in queryset._known_related_objects.items():
-                    # Avoid overwriting objects loaded e.g. by select_related
+            # Add the known related objects to the model.
+            for field, rel_objs, rel_getters in known_related_objects:
+                # Avoid overwriting objects loaded by, e.g., select_related().
                 if field.is_cached(obj):
                     continue
-                    pk = getattr(obj, field.get_attname())
+                rel_obj_id = tuple([rel_getter(obj) for rel_getter in rel_getters])
                 try:
-                        rel_obj = rel_objs[pk]
+                    rel_obj = rel_objs[rel_obj_id]
                 except KeyError:
-                        pass  # may happen in qs1 | qs2 scenarios
+                    pass  # May happen in qs1 | qs2 scenarios.
                 else:
                     setattr(obj, field.name, rel_obj)
 

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):

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)

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)