Fixed #15250 -- Avoided extra query on some multi-table inheritance queries.

Thanks marekw2143 for the initial patch and carljm for support.

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/model_inheritance_regress/tests.py b/tests/model_inheritance_regress/tests.py
index 7499e4e42f..d2a1fc29fd 100644
--- a/tests/model_inheritance_regress/tests.py
+++ b/tests/model_inheritance_regress/tests.py
@@ -496,3 +496,35 @@ class ModelInheritanceTest(TestCase):
                 r.supplier_set.all(),
                 [s], lambda x: x,
             )
+
+    def test_queries_on_parent_access(self):
+        italian_restaurant = ItalianRestaurant.objects.create(
+            name="Guido's House of Pasta",
+            address='944 W. Fullerton',
+            serves_hot_dogs=True,
+            serves_pizza=False,
+            serves_gnocchi=True,
+        )
+
+        # No queries are made when accessing the parent objects.
+        italian_restaurant = ItalianRestaurant.objects.get(pk=italian_restaurant.pk)
+        with self.assertNumQueries(0):
+            restaurant = italian_restaurant.restaurant_ptr
+            self.assertEqual(restaurant.place_ptr.restaurant, restaurant)
+            self.assertEqual(restaurant.italianrestaurant, italian_restaurant)
+
+        # One query is made when accessing the parent objects when the instance
+        # is deferred.
+        italian_restaurant = ItalianRestaurant.objects.only('serves_gnocchi').get(pk=italian_restaurant.pk)
+        with self.assertNumQueries(1):
+            restaurant = italian_restaurant.restaurant_ptr
+            self.assertEqual(restaurant.place_ptr.restaurant, restaurant)
+            self.assertEqual(restaurant.italianrestaurant, italian_restaurant)
+
+        # No queries are made when accessing the parent objects when the
+        # instance has deferred a field not present in the parent table.
+        italian_restaurant = ItalianRestaurant.objects.defer('serves_gnocchi').get(pk=italian_restaurant.pk)
+        with self.assertNumQueries(0):
+            restaurant = italian_restaurant.restaurant_ptr
+            self.assertEqual(restaurant.place_ptr.restaurant, restaurant)
+            self.assertEqual(restaurant.italianrestaurant, italian_restaurant)