django/tests/delete/tests.py

from math import ceil

from django.db import IntegrityError, connection, models
from django.db.models.deletion import (
    Collector, ProtectedError, RestrictedError,
)
from django.db.models.sql.constants import GET_ITERATOR_CHUNK_SIZE
from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature

from .models import (
    B1, B2, MR, A, Avatar, B, Base, Child, DeleteBottom, DeleteTop, GenericB1,
    GenericB2, GenericDeleteBottom, HiddenUser, HiddenUserProfile, M, M2MFrom,
    M2MTo, MRNull, Origin, P, Parent, R, RChild, RChildChild, Referrer, S, T,
    User, create_a, get_default_r,
)


class OnDeleteTests(TestCase):
    def setUp(self):
        self.DEFAULT = get_default_r()

    def test_auto(self):
        a = create_a('auto')
        a.auto.delete()
        self.assertFalse(A.objects.filter(name='auto').exists())

    def test_non_callable(self):
        msg = 'on_delete must be callable.'
        with self.assertRaisesMessage(TypeError, msg):
            models.ForeignKey('self', on_delete=None)
        with self.assertRaisesMessage(TypeError, msg):
            models.OneToOneField('self', on_delete=None)

    def test_auto_nullable(self):
        a = create_a('auto_nullable')
        a.auto_nullable.delete()
        self.assertFalse(A.objects.filter(name='auto_nullable').exists())

    def test_setvalue(self):
        a = create_a('setvalue')
        a.setvalue.delete()
        a = A.objects.get(pk=a.pk)
        self.assertEqual(self.DEFAULT, a.setvalue.pk)

    def test_setnull(self):
        a = create_a('setnull')
        a.setnull.delete()
        a = A.objects.get(pk=a.pk)
        self.assertIsNone(a.setnull)

    def test_setdefault(self):
        a = create_a('setdefault')
        a.setdefault.delete()
        a = A.objects.get(pk=a.pk)
        self.assertEqual(self.DEFAULT, a.setdefault.pk)

    def test_setdefault_none(self):
        a = create_a('setdefault_none')
        a.setdefault_none.delete()
        a = A.objects.get(pk=a.pk)
        self.assertIsNone(a.setdefault_none)

    def test_cascade(self):
        a = create_a('cascade')
        a.cascade.delete()
        self.assertFalse(A.objects.filter(name='cascade').exists())

    def test_cascade_nullable(self):
        a = create_a('cascade_nullable')
        a.cascade_nullable.delete()
        self.assertFalse(A.objects.filter(name='cascade_nullable').exists())

    def test_protect(self):
        a = create_a('protect')
        msg = (
            "Cannot delete some instances of model 'R' because they are "
            "referenced through protected foreign keys: 'A.protect'."
        )
        with self.assertRaisesMessage(IntegrityError, msg):
            a.protect.delete()

    def test_protect_multiple(self):
        a = create_a('protect')
        B.objects.create(protect=a.protect)
        msg = (
            "Cannot delete some instances of model 'R' because they are "
            "referenced through protected foreign keys: 'A.protect', "
            "'B.protect'."
        )
        with self.assertRaisesMessage(ProtectedError, msg):
            a.protect.delete()

    def test_do_nothing(self):
        # Testing DO_NOTHING is a bit harder: It would raise IntegrityError for a normal model,
        # so we connect to pre_delete and set the fk to a known value.
        replacement_r = R.objects.create()

        def check_do_nothing(sender, **kwargs):
            obj = kwargs['instance']
            obj.donothing_set.update(donothing=replacement_r)
        models.signals.pre_delete.connect(check_do_nothing)
        a = create_a('do_nothing')
        a.donothing.delete()
        a = A.objects.get(pk=a.pk)
        self.assertEqual(replacement_r, a.donothing)
        models.signals.pre_delete.disconnect(check_do_nothing)

    def test_do_nothing_qscount(self):
        """
        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()
        self.assertFalse(R.objects.filter(pk=child.pk).exists())

    def test_inheritance_cascade_down(self):
        child = RChild.objects.create()
        parent = child.r_ptr
        parent.delete()
        self.assertFalse(RChild.objects.filter(pk=child.pk).exists())

    def test_cascade_from_child(self):
        a = create_a('child')
        a.child.delete()
        self.assertFalse(A.objects.filter(name='child').exists())
        self.assertFalse(R.objects.filter(pk=a.child_id).exists())

    def test_cascade_from_parent(self):
        a = create_a('child')
        R.objects.get(pk=a.child_id).delete()
        self.assertFalse(A.objects.filter(name='child').exists())
        self.assertFalse(RChild.objects.filter(pk=a.child_id).exists())

    def test_setnull_from_child(self):
        a = create_a('child_setnull')
        a.child_setnull.delete()
        self.assertFalse(R.objects.filter(pk=a.child_setnull_id).exists())

        a = A.objects.get(pk=a.pk)
        self.assertIsNone(a.child_setnull)

    def test_setnull_from_parent(self):
        a = create_a('child_setnull')
        R.objects.get(pk=a.child_setnull_id).delete()
        self.assertFalse(RChild.objects.filter(pk=a.child_setnull_id).exists())

        a = A.objects.get(pk=a.pk)
        self.assertIsNone(a.child_setnull)

    def test_o2o_setnull(self):
        a = create_a('o2o_setnull')
        a.o2o_setnull.delete()
        a = A.objects.get(pk=a.pk)
        self.assertIsNone(a.o2o_setnull)

    def test_restrict(self):
        a = create_a('restrict')
        msg = (
            "Cannot delete some instances of model 'R' because they are "
            "referenced through a restricted foreign key: 'A.restrict'."
        )
        with self.assertRaisesMessage(RestrictedError, msg):
            a.restrict.delete()

    def test_restrict_path_cascade_indirect(self):
        a = create_a('restrict')
        a.restrict.p = P.objects.create()
        a.restrict.save()
        msg = (
            "Cannot delete some instances of model 'R' because they are "
            "referenced through a restricted foreign key: 'A.restrict'."
        )
        with self.assertRaisesMessage(RestrictedError, msg):
            a.restrict.p.delete()
        # Object referenced also with CASCADE relationship can be deleted.
        a.cascade.p = a.restrict.p
        a.cascade.save()
        a.restrict.p.delete()
        self.assertFalse(A.objects.filter(name='restrict').exists())
        self.assertFalse(R.objects.filter(pk=a.restrict_id).exists())

    def test_restrict_path_cascade_direct(self):
        a = create_a('restrict')
        a.restrict.p = P.objects.create()
        a.restrict.save()
        a.cascade_p = a.restrict.p
        a.save()
        a.restrict.p.delete()
        self.assertFalse(A.objects.filter(name='restrict').exists())
        self.assertFalse(R.objects.filter(pk=a.restrict_id).exists())

    def test_restrict_path_cascade_indirect_diamond(self):
        delete_top = DeleteTop.objects.create()
        b1 = B1.objects.create(delete_top=delete_top)
        b2 = B2.objects.create(delete_top=delete_top)
        DeleteBottom.objects.create(b1=b1, b2=b2)
        msg = (
            "Cannot delete some instances of model 'B1' because they are "
            "referenced through a restricted foreign key: 'DeleteBottom.b1'."
        )
        with self.assertRaisesMessage(RestrictedError, msg):
            b1.delete()
        self.assertTrue(DeleteTop.objects.exists())
        self.assertTrue(B1.objects.exists())
        self.assertTrue(B2.objects.exists())
        self.assertTrue(DeleteBottom.objects.exists())
        # Object referenced also with CASCADE relationship can be deleted.
        delete_top.delete()
        self.assertFalse(DeleteTop.objects.exists())
        self.assertFalse(B1.objects.exists())
        self.assertFalse(B2.objects.exists())
        self.assertFalse(DeleteBottom.objects.exists())

    def test_restrict_gfk_no_fast_delete(self):
        delete_top = DeleteTop.objects.create()
        generic_b1 = GenericB1.objects.create(generic_delete_top=delete_top)
        generic_b2 = GenericB2.objects.create(generic_delete_top=delete_top)
        GenericDeleteBottom.objects.create(generic_b1=generic_b1, generic_b2=generic_b2)
        msg = (
            "Cannot delete some instances of model 'GenericB1' because they "
            "are referenced through a restricted foreign key: "
            "'GenericDeleteBottom.generic_b1'."
        )
        with self.assertRaisesMessage(RestrictedError, msg):
            generic_b1.delete()
        self.assertTrue(DeleteTop.objects.exists())
        self.assertTrue(GenericB1.objects.exists())
        self.assertTrue(GenericB2.objects.exists())
        self.assertTrue(GenericDeleteBottom.objects.exists())
        # Object referenced also with CASCADE relationship can be deleted.
        delete_top.delete()
        self.assertFalse(DeleteTop.objects.exists())
        self.assertFalse(GenericB1.objects.exists())
        self.assertFalse(GenericB2.objects.exists())
        self.assertFalse(GenericDeleteBottom.objects.exists())


class DeletionTests(TestCase):

    def test_m2m(self):
        m = M.objects.create()
        r = R.objects.create()
        MR.objects.create(m=m, r=r)
        r.delete()
        self.assertFalse(MR.objects.exists())

        r = R.objects.create()
        MR.objects.create(m=m, r=r)
        m.delete()
        self.assertFalse(MR.objects.exists())

        m = M.objects.create()
        r = R.objects.create()
        m.m2m.add(r)
        r.delete()
        through = M._meta.get_field('m2m').remote_field.through
        self.assertFalse(through.objects.exists())

        r = R.objects.create()
        m.m2m.add(r)
        m.delete()
        self.assertFalse(through.objects.exists())

        m = M.objects.create()
        r = R.objects.create()
        MRNull.objects.create(m=m, r=r)
        r.delete()
        self.assertFalse(not MRNull.objects.exists())
        self.assertFalse(m.m2m_through_null.exists())

    def test_bulk(self):
        s = S.objects.create(r=R.objects.create())
        for i in range(2 * GET_ITERATOR_CHUNK_SIZE):
            T.objects.create(s=s)
        #   1 (select related `T` instances)
        # + 1 (select related `U` instances)
        # + 2 (delete `T` instances in batches)
        # + 1 (delete `s`)
        self.assertNumQueries(5, s.delete)
        self.assertFalse(S.objects.exists())

    def test_instance_update(self):
        deleted = []
        related_setnull_sets = []

        def pre_delete(sender, **kwargs):
            obj = kwargs['instance']
            deleted.append(obj)
            if isinstance(obj, R):
                related_setnull_sets.append([a.pk for a in obj.setnull_set.all()])

        models.signals.pre_delete.connect(pre_delete)
        a = create_a('update_setnull')
        a.setnull.delete()

        a = create_a('update_cascade')
        a.cascade.delete()

        for obj in deleted:
            self.assertIsNone(obj.pk)

        for pk_list in related_setnull_sets:
            for a in A.objects.filter(id__in=pk_list):
                self.assertIsNone(a.setnull)

        models.signals.pre_delete.disconnect(pre_delete)

    def test_deletion_order(self):
        pre_delete_order = []
        post_delete_order = []

        def log_post_delete(sender, **kwargs):
            pre_delete_order.append((sender, kwargs['instance'].pk))

        def log_pre_delete(sender, **kwargs):
            post_delete_order.append((sender, kwargs['instance'].pk))

        models.signals.post_delete.connect(log_post_delete)
        models.signals.pre_delete.connect(log_pre_delete)

        r = R.objects.create(pk=1)
        s1 = S.objects.create(pk=1, r=r)
        s2 = S.objects.create(pk=2, r=r)
        T.objects.create(pk=1, s=s1)
        T.objects.create(pk=2, s=s2)
        RChild.objects.create(r_ptr=r)
        r.delete()
        self.assertEqual(
            pre_delete_order, [(T, 2), (T, 1), (RChild, 1), (S, 2), (S, 1), (R, 1)]
        )
        self.assertEqual(
            post_delete_order, [(T, 1), (T, 2), (RChild, 1), (S, 1), (S, 2), (R, 1)]
        )

        models.signals.post_delete.disconnect(log_post_delete)
        models.signals.pre_delete.disconnect(log_pre_delete)

    def test_relational_post_delete_signals_happen_before_parent_object(self):
        deletions = []

        def log_post_delete(instance, **kwargs):
            self.assertTrue(R.objects.filter(pk=instance.r_id))
            self.assertIs(type(instance), S)
            deletions.append(instance.id)

        r = R.objects.create(pk=1)
        S.objects.create(pk=1, r=r)

        models.signals.post_delete.connect(log_post_delete, sender=S)

        try:
            r.delete()
        finally:
            models.signals.post_delete.disconnect(log_post_delete)

        self.assertEqual(len(deletions), 1)
        self.assertEqual(deletions[0], 1)

    @skipUnlessDBFeature("can_defer_constraint_checks")
    def test_can_defer_constraint_checks(self):
        u = User.objects.create(
            avatar=Avatar.objects.create()
        )
        a = Avatar.objects.get(pk=u.avatar_id)
        # 1 query to find the users for the avatar.
        # 1 query to delete the user
        # 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.assertEqual(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
        # 1 query to delete the avatar
        self.assertNumQueries(4, a.delete)
        self.assertFalse(User.objects.exists())
        self.assertFalse(Avatar.objects.exists())
        self.assertEqual(len(calls), 1)
        models.signals.post_delete.disconnect(noop, sender=User)

    def test_hidden_related(self):
        r = R.objects.create()
        h = HiddenUser.objects.create(r=r)
        HiddenUserProfile.objects.create(user=h)

        r.delete()
        self.assertEqual(HiddenUserProfile.objects.count(), 0)

    def test_large_delete(self):
        TEST_SIZE = 2000
        objs = [Avatar() for i in range(0, TEST_SIZE)]
        Avatar.objects.bulk_create(objs)
        # Calculate the number of queries needed.
        batch_size = connection.ops.bulk_batch_size(['pk'], objs)
        # The related fetches are done in batches.
        batches = ceil(len(objs) / batch_size)
        # One query for Avatar.objects.all() and then one related fast delete for
        # each batch.
        fetches_to_mem = 1 + batches
        # The Avatar objects are going to be deleted in batches of GET_ITERATOR_CHUNK_SIZE
        queries = fetches_to_mem + TEST_SIZE // GET_ITERATOR_CHUNK_SIZE
        self.assertNumQueries(queries, Avatar.objects.all().delete)
        self.assertFalse(Avatar.objects.exists())

    def test_large_delete_related(self):
        TEST_SIZE = 2000
        s = S.objects.create(r=R.objects.create())
        for i in range(TEST_SIZE):
            T.objects.create(s=s)

        batch_size = max(connection.ops.bulk_batch_size(['pk'], range(TEST_SIZE)), 1)

        # TEST_SIZE / batch_size (select related `T` instances)
        # + 1 (select related `U` instances)
        # + TEST_SIZE / GET_ITERATOR_CHUNK_SIZE (delete `T` instances in batches)
        # + 1 (delete `s`)
        expected_num_queries = ceil(TEST_SIZE / batch_size)
        expected_num_queries += ceil(TEST_SIZE / GET_ITERATOR_CHUNK_SIZE) + 2

        self.assertNumQueries(expected_num_queries, s.delete)
        self.assertFalse(S.objects.exists())
        self.assertFalse(T.objects.exists())

    def test_delete_with_keeping_parents(self):
        child = RChild.objects.create()
        parent_id = child.r_ptr_id
        child.delete(keep_parents=True)
        self.assertFalse(RChild.objects.filter(id=child.id).exists())
        self.assertTrue(R.objects.filter(id=parent_id).exists())

    def test_delete_with_keeping_parents_relationships(self):
        child = RChild.objects.create()
        parent_id = child.r_ptr_id
        parent_referent_id = S.objects.create(r=child.r_ptr).pk
        child.delete(keep_parents=True)
        self.assertFalse(RChild.objects.filter(id=child.id).exists())
        self.assertTrue(R.objects.filter(id=parent_id).exists())
        self.assertTrue(S.objects.filter(pk=parent_referent_id).exists())

        childchild = RChildChild.objects.create()
        parent_id = childchild.rchild_ptr.r_ptr_id
        child_id = childchild.rchild_ptr_id
        parent_referent_id = S.objects.create(r=childchild.rchild_ptr.r_ptr).pk
        childchild.delete(keep_parents=True)
        self.assertFalse(RChildChild.objects.filter(id=childchild.id).exists())
        self.assertTrue(RChild.objects.filter(id=child_id).exists())
        self.assertTrue(R.objects.filter(id=parent_id).exists())
        self.assertTrue(S.objects.filter(pk=parent_referent_id).exists())

    def test_queryset_delete_returns_num_rows(self):
        """
        QuerySet.delete() should return the number of deleted rows and a
        dictionary with the number of deletions for each object type.
        """
        Avatar.objects.bulk_create([Avatar(desc='a'), Avatar(desc='b'), Avatar(desc='c')])
        avatars_count = Avatar.objects.count()
        deleted, rows_count = Avatar.objects.all().delete()
        self.assertEqual(deleted, avatars_count)

        # more complex example with multiple object types
        r = R.objects.create()
        h1 = HiddenUser.objects.create(r=r)
        HiddenUser.objects.create(r=r)
        HiddenUserProfile.objects.create(user=h1)
        existed_objs = {
            R._meta.label: R.objects.count(),
            HiddenUser._meta.label: HiddenUser.objects.count(),
            A._meta.label: A.objects.count(),
            MR._meta.label: MR.objects.count(),
            HiddenUserProfile._meta.label: HiddenUserProfile.objects.count(),
        }
        deleted, deleted_objs = R.objects.all().delete()
        for k, v in existed_objs.items():
            self.assertEqual(deleted_objs[k], v)

    def test_model_delete_returns_num_rows(self):
        """
        Model.delete() should return the number of deleted rows and a
        dictionary with the number of deletions for each object type.
        """
        r = R.objects.create()
        h1 = HiddenUser.objects.create(r=r)
        h2 = HiddenUser.objects.create(r=r)
        HiddenUser.objects.create(r=r)
        HiddenUserProfile.objects.create(user=h1)
        HiddenUserProfile.objects.create(user=h2)
        m1 = M.objects.create()
        m2 = M.objects.create()
        MR.objects.create(r=r, m=m1)
        r.m_set.add(m1)
        r.m_set.add(m2)
        r.save()
        existed_objs = {
            R._meta.label: R.objects.count(),
            HiddenUser._meta.label: HiddenUser.objects.count(),
            A._meta.label: A.objects.count(),
            MR._meta.label: MR.objects.count(),
            HiddenUserProfile._meta.label: HiddenUserProfile.objects.count(),
            M.m2m.through._meta.label: M.m2m.through.objects.count(),
        }
        deleted, deleted_objs = r.delete()
        self.assertEqual(deleted, sum(existed_objs.values()))
        for k, v in existed_objs.items():
            self.assertEqual(deleted_objs[k], v)

    def test_proxied_model_duplicate_queries(self):
        """
        #25685 - Deleting instances of a model with existing proxy
        classes should not issue multiple queries during cascade
        deletion of referring models.
        """
        avatar = Avatar.objects.create()
        # One query for the Avatar table and a second for the User one.
        with self.assertNumQueries(2):
            avatar.delete()

    def test_only_referenced_fields_selected(self):
        """
        Only referenced fields are selected during cascade deletion SELECT
        unless deletion signals are connected.
        """
        origin = Origin.objects.create()
        expected_sql = str(
            Referrer.objects.only(
                # Both fields are referenced by SecondReferrer.
                'id', 'unique_field',
            ).filter(origin__in=[origin]).query
        )
        with self.assertNumQueries(2) as ctx:
            origin.delete()
        self.assertEqual(ctx.captured_queries[0]['sql'], expected_sql)

        def receiver(instance, **kwargs):
            pass

        # All fields are selected if deletion signals are connected.
        for signal_name in ('pre_delete', 'post_delete'):
            with self.subTest(signal=signal_name):
                origin = Origin.objects.create()
                signal = getattr(models.signals, signal_name)
                signal.connect(receiver, sender=Referrer)
                with self.assertNumQueries(2) as ctx:
                    origin.delete()
                self.assertIn(
                    connection.ops.quote_name('large_field'),
                    ctx.captured_queries[0]['sql'],
                )
                signal.disconnect(receiver, sender=Referrer)


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.assertEqual(User.objects.count(), 1)
        self.assertTrue(User.objects.filter(pk=u2.pk).exists())

    def test_fast_delete_instance_set_pk_none(self):
        u = User.objects.create()
        # User can be fast-deleted.
        collector = Collector(using='default')
        self.assertTrue(collector.can_fast_delete(u))
        u.delete()
        self.assertIsNone(u.pk)

    def test_fast_delete_joined_qs(self):
        a = Avatar.objects.create(desc='a')
        User.objects.create(avatar=a)
        u2 = User.objects.create()
        self.assertNumQueries(1, User.objects.filter(avatar__desc='a').delete)
        self.assertEqual(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
        self.assertNumQueries(2, c.delete)
        self.assertFalse(Child.objects.exists())
        self.assertEqual(Parent.objects.count(), 1)
        self.assertEqual(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())

    def test_fast_delete_large_batch(self):
        User.objects.bulk_create(User() for i in range(0, 2000))
        # No problems here - we aren't going to cascade, so we will fast
        # delete the objects in a single query.
        self.assertNumQueries(1, User.objects.all().delete)
        a = Avatar.objects.create(desc='a')
        User.objects.bulk_create(User(avatar=a) for i in range(0, 2000))
        # We don't hit parameter amount limits for a, so just one query for
        # that + fast delete of the related objs.
        self.assertNumQueries(2, a.delete)
        self.assertEqual(User.objects.count(), 0)

    def test_fast_delete_empty_no_update_can_self_select(self):
        """
        #25932 - Fast deleting on backends that don't have the
        `no_update_can_self_select` feature should work even if the specified
        filter doesn't match any row.
        """
        with self.assertNumQueries(1):
            self.assertEqual(
                User.objects.filter(avatar__desc='missing').delete(),
                (0, {'delete.User': 0})
            )

    def test_fast_delete_combined_relationships(self):
        # The cascading fast-delete of SecondReferrer should be combined
        # in a single DELETE WHERE referrer_id OR unique_field.
        origin = Origin.objects.create()
        referer = Referrer.objects.create(origin=origin, unique_field=42)
        with self.assertNumQueries(2):
            referer.delete()