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.

6 files changed, 148 insertions, 12 deletions

diff --git a/tests/modeltests/delete/models.py b/tests/modeltests/delete/models.py
index e0cec426ea..65d4e6f725 100644
--- a/tests/modeltests/delete/models.py
+++ b/tests/modeltests/delete/models.py
@@ -95,7 +95,7 @@ class MRNull(models.Model):
 
 
 class Avatar(models.Model):
-    pass
+    desc = models.TextField(null=True)
 
 
 class User(models.Model):
@@ -108,3 +108,21 @@ class HiddenUser(models.Model):
 
 class HiddenUserProfile(models.Model):
     user = models.ForeignKey(HiddenUser)
+
+class M2MTo(models.Model):
+    pass
+
+class M2MFrom(models.Model):
+    m2m = models.ManyToManyField(M2MTo)
+
+class Parent(models.Model):
+    pass
+
+class Child(Parent):
+    pass
+
+class Base(models.Model):
+    pass
+
+class RelToBase(models.Model):
+    base = models.ForeignKey(Base, on_delete=models.DO_NOTHING)
diff --git a/tests/modeltests/delete/tests.py b/tests/modeltests/delete/tests.py
index 26f2fd52c1..2610cb4b39 100644
--- a/tests/modeltests/delete/tests.py
+++ b/tests/modeltests/delete/tests.py
@@ -1,11 +1,12 @@
 from __future__ import absolute_import
 
-from django.db import models, IntegrityError
+from django.db import models, IntegrityError, connection
 from django.test import TestCase, skipUnlessDBFeature, skipIfDBFeature
 from django.utils.six.moves import xrange
 
 from .models import (R, RChild, S, T, U, A, M, MR, MRNull,
-    create_a, get_default_r, User, Avatar, HiddenUser, HiddenUserProfile)
+    create_a, get_default_r, User, Avatar, HiddenUser, HiddenUserProfile,
+    M2MTo, M2MFrom, Parent, Child, Base)
 
 
 class OnDeleteTests(TestCase):
@@ -74,6 +75,16 @@ class OnDeleteTests(TestCase):
         self.assertEqual(replacement_r, a.donothing)
         models.signals.pre_delete.disconnect(check_do_nothing)
 
+    def test_do_nothing_qscount(self):
+        """
+        Test that a models.DO_NOTHING relation doesn't trigger a query.
+        """
+        b = Base.objects.create()
+        with self.assertNumQueries(1):
+            # RelToBase should not be queried.
+            b.delete()
+        self.assertEqual(Base.objects.count(), 0)
+
     def test_inheritance_cascade_up(self):
         child = RChild.objects.create()
         child.delete()
@@ -229,16 +240,34 @@ class DeletionTests(TestCase):
         # 1 query to delete the avatar
         # The important thing is that when we can defer constraint checks there
         # is no need to do an UPDATE on User.avatar to null it out.
+
+        # Attach a signal to make sure we will not do fast_deletes.
+        calls = []
+        def noop(*args, **kwargs):
+            calls.append('')
+        models.signals.post_delete.connect(noop, sender=User)
+
         self.assertNumQueries(3, a.delete)
         self.assertFalse(User.objects.exists())
         self.assertFalse(Avatar.objects.exists())
+        self.assertEquals(len(calls), 1)
+        models.signals.post_delete.disconnect(noop, sender=User)
 
     @skipIfDBFeature("can_defer_constraint_checks")
     def test_cannot_defer_constraint_checks(self):
         u = User.objects.create(
             avatar=Avatar.objects.create()
         )
+        # Attach a signal to make sure we will not do fast_deletes.
+        calls = []
+        def noop(*args, **kwargs):
+            calls.append('')
+        models.signals.post_delete.connect(noop, sender=User)
+
         a = Avatar.objects.get(pk=u.avatar_id)
+        # The below doesn't make sense... Why do we need to null out
+        # user.avatar if we are going to delete the user immediately after it,
+        # and there are no more cascades.
         # 1 query to find the users for the avatar.
         # 1 query to delete the user
         # 1 query to null out user.avatar, because we can't defer the constraint
@@ -246,6 +275,8 @@ class DeletionTests(TestCase):
         self.assertNumQueries(4, a.delete)
         self.assertFalse(User.objects.exists())
         self.assertFalse(Avatar.objects.exists())
+        self.assertEquals(len(calls), 1)
+        models.signals.post_delete.disconnect(noop, sender=User)
 
     def test_hidden_related(self):
         r = R.objects.create()
@@ -254,3 +285,69 @@ class DeletionTests(TestCase):
 
         r.delete()
         self.assertEqual(HiddenUserProfile.objects.count(), 0)
+
+class FastDeleteTests(TestCase):
+
+    def test_fast_delete_fk(self):
+        u = User.objects.create(
+            avatar=Avatar.objects.create()
+        )
+        a = Avatar.objects.get(pk=u.avatar_id)
+        # 1 query to fast-delete the user
+        # 1 query to delete the avatar
+        self.assertNumQueries(2, a.delete)
+        self.assertFalse(User.objects.exists())
+        self.assertFalse(Avatar.objects.exists())
+
+    def test_fast_delete_m2m(self):
+        t = M2MTo.objects.create()
+        f = M2MFrom.objects.create()
+        f.m2m.add(t)
+        # 1 to delete f, 1 to fast-delete m2m for f
+        self.assertNumQueries(2, f.delete)
+
+    def test_fast_delete_revm2m(self):
+        t = M2MTo.objects.create()
+        f = M2MFrom.objects.create()
+        f.m2m.add(t)
+        # 1 to delete t, 1 to fast-delete t's m_set
+        self.assertNumQueries(2, f.delete)
+
+    def test_fast_delete_qs(self):
+        u1 = User.objects.create()
+        u2 = User.objects.create()
+        self.assertNumQueries(1, User.objects.filter(pk=u1.pk).delete)
+        self.assertEquals(User.objects.count(), 1)
+        self.assertTrue(User.objects.filter(pk=u2.pk).exists())
+
+    def test_fast_delete_joined_qs(self):
+        a = Avatar.objects.create(desc='a')
+        User.objects.create(avatar=a)
+        u2 = User.objects.create()
+        expected_queries = 1 if connection.features.update_can_self_select else 2
+        self.assertNumQueries(expected_queries,
+                              User.objects.filter(avatar__desc='a').delete)
+        self.assertEquals(User.objects.count(), 1)
+        self.assertTrue(User.objects.filter(pk=u2.pk).exists())
+
+    def test_fast_delete_inheritance(self):
+        c = Child.objects.create()
+        p = Parent.objects.create()
+        # 1 for self, 1 for parent
+        # However, this doesn't work as child.parent access creates a query,
+        # and this means we will be generating extra queries (a lot for large
+        # querysets). This is not a fast-delete problem.
+        # self.assertNumQueries(2, c.delete)
+        c.delete()
+        self.assertFalse(Child.objects.exists())
+        self.assertEquals(Parent.objects.count(), 1)
+        self.assertEquals(Parent.objects.filter(pk=p.pk).count(), 1)
+        # 1 for self delete, 1 for fast delete of empty "child" qs.
+        self.assertNumQueries(2, p.delete)
+        self.assertFalse(Parent.objects.exists())
+        # 1 for self delete, 1 for fast delete of empty "child" qs.
+        c = Child.objects.create()
+        p = c.parent_ptr
+        self.assertNumQueries(2, p.delete)
+        self.assertFalse(Parent.objects.exists())
+        self.assertFalse(Child.objects.exists())
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):