Fixed #18676 -- Allow fast-path deletion of objects

Objects can be fast-path deleted if there are no signals, and there are
no further cascades. If fast-path is taken, the objects do not need to
be loaded into memory before deletion.

Thanks to Jeremy Dunck, Simon Charette and Alex Gaynor for reviewing
the patch.

4 files changed, 30 insertions, 9 deletions

diff --git a/tests/regressiontests/admin_util/models.py b/tests/regressiontests/admin_util/models.py
index b3504a1fa4..32a6cd6291 100644
--- a/tests/regressiontests/admin_util/models.py
+++ b/tests/regressiontests/admin_util/models.py
@@ -39,3 +39,6 @@ class Guest(models.Model):
 
     class Meta:
         verbose_name = "awesome guest"
+
+class EventGuide(models.Model):
+    event = models.ForeignKey(Event, on_delete=models.DO_NOTHING)
diff --git a/tests/regressiontests/admin_util/tests.py b/tests/regressiontests/admin_util/tests.py
index d04740ce95..ef8a91d1db 100644
--- a/tests/regressiontests/admin_util/tests.py
+++ b/tests/regressiontests/admin_util/tests.py
@@ -17,7 +17,7 @@ from django.utils.formats import localize
 from django.utils.safestring import mark_safe
 from django.utils import six
 
-from .models import Article, Count, Event, Location
+from .models import Article, Count, Event, Location, EventGuide
 
 
 class NestedObjectsTests(TestCase):
@@ -71,6 +71,17 @@ class NestedObjectsTests(TestCase):
         # Should not require additional queries to populate the nested graph.
         self.assertNumQueries(2, self._collect, 0)
 
+    def test_on_delete_do_nothing(self):
+        """
+        Check that the nested collector doesn't query for DO_NOTHING objects.
+        """
+        n = NestedObjects(using=DEFAULT_DB_ALIAS)
+        objs = [Event.objects.create()]
+        EventGuide.objects.create(event=objs[0])
+        with self.assertNumQueries(2):
+            # One for Location, one for Guest, and no query for EventGuide
+            n.collect(objs)
+
 class UtilTests(unittest.TestCase):
     def test_values_from_lookup_field(self):
         """
diff --git a/tests/regressiontests/delete_regress/tests.py b/tests/regressiontests/delete_regress/tests.py
index 32feae2ded..ebe59bffd7 100644
--- a/tests/regressiontests/delete_regress/tests.py
+++ b/tests/regressiontests/delete_regress/tests.py
@@ -3,7 +3,7 @@ from __future__ import absolute_import
 import datetime
 
 from django.conf import settings
-from django.db import backend, transaction, DEFAULT_DB_ALIAS
+from django.db import backend, transaction, DEFAULT_DB_ALIAS, models
 from django.test import TestCase, TransactionTestCase, skipUnlessDBFeature
 
 from .models import (Book, Award, AwardNote, Person, Child, Toy, PlayedWith,
@@ -139,17 +139,24 @@ class DeleteCascadeTransactionTests(TransactionTestCase):
         eaten = Eaten.objects.create(food=apple, meal="lunch")
 
         apple.delete()
+        self.assertFalse(Food.objects.exists())
+        self.assertFalse(Eaten.objects.exists())
+
 
 class LargeDeleteTests(TestCase):
     def test_large_deletes(self):
         "Regression for #13309 -- if the number of objects > chunk size, deletion still occurs"
         for x in range(300):
             track = Book.objects.create(pagecount=x+100)
+        # attach a signal to make sure we will not fast-delete
+        def noop(*args, **kwargs):
+            pass
+        models.signals.post_delete.connect(noop, sender=Book)
         Book.objects.all().delete()
+        models.signals.post_delete.disconnect(noop, sender=Book)
         self.assertEqual(Book.objects.count(), 0)
 
 
-
 class ProxyDeleteTest(TestCase):
     """
     Tests on_delete behavior for proxy models.
diff --git a/tests/regressiontests/dispatch/tests/test_dispatcher.py b/tests/regressiontests/dispatch/tests/test_dispatcher.py
index 4e4669d34c..5f8f92acaf 100644
--- a/tests/regressiontests/dispatch/tests/test_dispatcher.py
+++ b/tests/regressiontests/dispatch/tests/test_dispatcher.py
@@ -127,15 +127,15 @@ class DispatcherTests(unittest.TestCase):
         self._testIsClean(a_signal)
 
     def test_has_listeners(self):
-        self.assertIs(a_signal.has_listeners(), False)
-        self.assertIs(a_signal.has_listeners(sender=object()), False)
+        self.assertFalse(a_signal.has_listeners())
+        self.assertFalse(a_signal.has_listeners(sender=object()))
         receiver_1 = Callable()
         a_signal.connect(receiver_1)
-        self.assertIs(a_signal.has_listeners(), True)
-        self.assertIs(a_signal.has_listeners(sender=object()), True)
+        self.assertTrue(a_signal.has_listeners())
+        self.assertTrue(a_signal.has_listeners(sender=object()))
         a_signal.disconnect(receiver_1)
-        self.assertIs(a_signal.has_listeners(), False)
-        self.assertIs(a_signal.has_listeners(sender=object()), False)
+        self.assertFalse(a_signal.has_listeners())
+        self.assertFalse(a_signal.has_listeners(sender=object()))
 
 
 class ReceiverTestCase(unittest.TestCase):