Fixed #19720 -- Oracle ordering related delete regression

When a query had a complex where condition (a condition targeting more
than the base table) a subquery was used for deletion. However, the
query had default ordering from the model's meta and Oracle doesn't
work with ordered subqueries.

The regression was caused by fast-path deletion code introduced in
1cd6e04 for fixing #18676.

Thanks to Dylan Klomparens for the report.

django/db/models/query.py

@@ -539,7 +539,7 @@ def delete(self):
         # Disable non-supported fields.
         del_query.query.select_for_update = False
         del_query.query.select_related = False
-        del_query.query.clear_ordering()
+        del_query.query.clear_ordering(force_empty=True)
 
         collector = Collector(using=del_query.db)
         collector.collect(del_query)

tests/regressiontests/delete_regress/models.py

@@ -99,3 +99,13 @@ class OrgUnit(models.Model):
 class Login(models.Model):
     description = models.CharField(max_length=32)
     orgunit = models.ForeignKey(OrgUnit)
+
+class House(models.Model):
+    address = models.CharField(max_length=32)
+
+class OrderedPerson(models.Model):
+    name = models.CharField(max_length=32)
+    lives_in = models.ForeignKey(House)
+
+    class Meta:
+        ordering = ['name']

tests/regressiontests/delete_regress/tests.py

@@ -10,7 +10,7 @@
 from .models import (Book, Award, AwardNote, Person, Child, Toy, PlayedWith,
     PlayedWithNote, Email, Researcher, Food, Eaten, Policy, Version, Location,
     Item, Image, File, Photo, FooFile, FooImage, FooPhoto, FooFileProxy, Login,
-    OrgUnit)
+    OrgUnit, OrderedPerson, House)
 
 
 # Can't run this test under SQLite, because you can't
@@ -347,3 +347,14 @@ def test_ticket_19102_defer(self):
         self.assertFalse(Login.objects.filter(pk=self.l1.pk).exists())
         self.assertTrue(Login.objects.filter(pk=self.l2.pk).exists())
 
+
+class OrderedDeleteTests(TestCase):
+    def test_meta_ordered_delete(self):
+        # When a subquery is performed by deletion code, the subquery must be
+        # cleared of all ordering. There was a but that caused _meta ordering
+        # to be used. Refs #19720.
+        h = House.objects.create(address='Foo')
+        OrderedPerson.objects.create(name='Jack', lives_in=h)
+        OrderedPerson.objects.create(name='Bob', lives_in=h)
+        OrderedPerson.objects.filter(lives_in__address='Foo').delete()
+        self.assertEqual(OrderedPerson.objects.count(), 0)