[1.11.x] Fixed #28166 -- Fixed Model._state.db on MTI parent model after saving child model.

Regression in 38575b007a.

Backport of 584e10c76e from master

django/db/models/fields/related_descriptors.py

@@ -288,7 +288,9 @@ class ForwardOneToOneDescriptor(ForwardManyToOneDescriptor):
             # on the related model for every deferred field.
             if not any(field in fields for field in deferred):
                 kwargs = {field: getattr(instance, field) for field in fields}
-                return rel_model(**kwargs)
+                obj = rel_model(**kwargs)
+                obj._state.db = instance._state.db
+                return obj
         return super(ForwardOneToOneDescriptor, self).get_object(instance)
 
 

docs/releases/1.11.1.txt

@@ -84,3 +84,7 @@ Bugfixes
 
 * Fixed crash in  ``BaseInlineFormSet._construct_form()`` when using
   ``save_as_new`` (:ticket:`28159`).
+
+* Fixed a regression where ``Model._state.db`` wasn't set correctly on
+  multi-table inheritance parent models after saving a child model
+  (:ticket:`28166`).

tests/model_inheritance_regress/tests.py

@@ -474,6 +474,10 @@ class ModelInheritanceTest(TestCase):
         jane = Supplier.objects.order_by("name").select_related("restaurant")[0]
         self.assertEqual(jane.restaurant.name, "Craft")
 
+    def test_ptr_accessor_assigns_db(self):
+        r = Restaurant.objects.create()
+        self.assertEqual(r.place_ptr._state.db, 'default')
+
     def test_related_filtering_query_efficiency_ticket_15844(self):
         r = Restaurant.objects.create(
             name="Guido's House of Pasta",