Fixed #17502 -- Made joining in inheritance cases consistent

The original problem was that when filtering two levels up in
inheritance chain, Django optimized the join generation so that the
middle model was skipped. But then Django generated joins from top
to middle to bottom for SELECT clause, and thus there was one extra
join (top->middle->bottom + top -> bottom).

This case is fixed in master as the filtering optimization is gone.
This has the side effect that in some cases there is still extra join
if the SELECT clause doesn't contain anything from middle or bottom.

tests/model_inheritance_regress/tests.py

@@ -8,6 +8,7 @@ from operator import attrgetter
 from django import forms
 
 from django.test import TestCase
+from django.utils.unittest import expectedFailure
 
 from .models import (Place, Restaurant, ItalianRestaurant, ParkingLot,
     ParkingLot2, ParkingLot3, Supplier, Wholesaler, Child, SelfRefParent,
@@ -422,3 +423,19 @@ class ModelInheritanceTest(TestCase):
         form = ProfileForm({'username': "user_with_profile", 'extra': "hello"},
                            instance=p)
         self.assertTrue(form.is_valid())
+
+    def test_inheritance_joins(self):
+        # Test for #17502 - check that filtering through two levels of
+        # inheritance chain doesn't generate extra joins.
+        qs = ItalianRestaurant.objects.all()
+        self.assertEqual(str(qs.query).count('JOIN'), 2)
+        qs = ItalianRestaurant.objects.filter(name='foo')
+        self.assertEqual(str(qs.query).count('JOIN'), 2)
+
+    @expectedFailure
+    def test_inheritance_values_joins(self):
+        # It would be nice (but not too important) to skip the middle join in
+        # this case. Skipping is possible as nothing from the middle model is
+        # used in the qs and top contains direct pointer to the bottom model.
+        qs = ItalianRestaurant.objects.values_list('serves_gnocchi').filter(name='foo')
+        self.assertEqual(str(qs.query).count('JOIN'), 1)