django1/tests/many_to_one/tests.py

import datetime
from copy import deepcopy

from django.core.exceptions import FieldError, MultipleObjectsReturned
from django.db import IntegrityError, models, transaction
from django.test import TestCase
from django.utils.translation import gettext_lazy

from .models import (
    Article, Category, Child, ChildNullableParent, City, Country, District,
    First, Parent, Record, Relation, Reporter, School, Student, Third,
    ToFieldChild,
)


class ManyToOneTests(TestCase):
    @classmethod
    def setUpTestData(cls):
        # Create a few Reporters.
        cls.r = Reporter(first_name='John', last_name='Smith', email='john@example.com')
        cls.r.save()
        cls.r2 = Reporter(first_name='Paul', last_name='Jones', email='paul@example.com')
        cls.r2.save()
        # Create an Article.
        cls.a = Article(headline='This is a test', pub_date=datetime.date(2005, 7, 27), reporter=cls.r)
        cls.a.save()

    def test_get(self):
        # Article objects have access to their related Reporter objects.
        r = self.a.reporter
        self.assertEqual(r.id, self.r.id)
        self.assertEqual((r.first_name, self.r.last_name), ('John', 'Smith'))

    def test_create(self):
        # You can also instantiate an Article by passing the Reporter's ID
        # instead of a Reporter object.
        a3 = Article(headline="Third article", pub_date=datetime.date(2005, 7, 27), reporter_id=self.r.id)
        a3.save()
        self.assertEqual(a3.reporter.id, self.r.id)

        # Similarly, the reporter ID can be a string.
        a4 = Article(headline="Fourth article", pub_date=datetime.date(2005, 7, 27), reporter_id=str(self.r.id))
        a4.save()
        self.assertEqual(repr(a4.reporter), "<Reporter: John Smith>")

    def test_add(self):
        # Create an Article via the Reporter object.
        new_article = self.r.article_set.create(headline="John's second story", pub_date=datetime.date(2005, 7, 29))
        self.assertEqual(repr(new_article), "<Article: John's second story>")
        self.assertEqual(new_article.reporter.id, self.r.id)

        # Create a new article, and add it to the article set.
        new_article2 = Article(headline="Paul's story", pub_date=datetime.date(2006, 1, 17))
        msg = "<Article: Paul's story> instance isn't saved. Use bulk=False or save the object first."
        with self.assertRaisesMessage(ValueError, msg):
            self.r.article_set.add(new_article2)

        self.r.article_set.add(new_article2, bulk=False)
        self.assertEqual(new_article2.reporter.id, self.r.id)
        self.assertSequenceEqual(
            self.r.article_set.all(),
            [new_article, new_article2, self.a],
        )

        # Add the same article to a different article set - check that it moves.
        self.r2.article_set.add(new_article2)
        self.assertEqual(new_article2.reporter.id, self.r2.id)
        self.assertSequenceEqual(self.r2.article_set.all(), [new_article2])

        # Adding an object of the wrong type raises TypeError.
        with transaction.atomic():
            with self.assertRaisesMessage(TypeError, "'Article' instance expected, got <Reporter:"):
                self.r.article_set.add(self.r2)
        self.assertSequenceEqual(
            self.r.article_set.all(),
            [new_article, self.a],
        )

    def test_set(self):
        new_article = self.r.article_set.create(headline="John's second story", pub_date=datetime.date(2005, 7, 29))
        new_article2 = self.r2.article_set.create(headline="Paul's story", pub_date=datetime.date(2006, 1, 17))

        # Assign the article to the reporter.
        new_article2.reporter = self.r
        new_article2.save()
        self.assertEqual(repr(new_article2.reporter), "<Reporter: John Smith>")
        self.assertEqual(new_article2.reporter.id, self.r.id)
        self.assertSequenceEqual(
            self.r.article_set.all(),
            [new_article, new_article2, self.a],
        )
        self.assertSequenceEqual(self.r2.article_set.all(), [])

        # Set the article back again.
        self.r2.article_set.set([new_article, new_article2])
        self.assertSequenceEqual(self.r.article_set.all(), [self.a])
        self.assertSequenceEqual(
            self.r2.article_set.all(),
            [new_article, new_article2],
        )

        # Funny case - because the ForeignKey cannot be null,
        # existing members of the set must remain.
        self.r.article_set.set([new_article])
        self.assertSequenceEqual(
            self.r.article_set.all(),
            [new_article, self.a],
        )
        self.assertSequenceEqual(self.r2.article_set.all(), [new_article2])

    def test_reverse_assignment_deprecation(self):
        msg = (
            "Direct assignment to the reverse side of a related set is "
            "prohibited. Use article_set.set() instead."
        )
        with self.assertRaisesMessage(TypeError, msg):
            self.r2.article_set = []

    def test_assign(self):
        new_article = self.r.article_set.create(headline="John's second story", pub_date=datetime.date(2005, 7, 29))
        new_article2 = self.r2.article_set.create(headline="Paul's story", pub_date=datetime.date(2006, 1, 17))

        # Assign the article to the reporter directly using the descriptor.
        new_article2.reporter = self.r
        new_article2.save()
        self.assertEqual(repr(new_article2.reporter), "<Reporter: John Smith>")
        self.assertEqual(new_article2.reporter.id, self.r.id)
        self.assertSequenceEqual(
            self.r.article_set.all(),
            [new_article, new_article2, self.a],
        )
        self.assertSequenceEqual(self.r2.article_set.all(), [])

        # Set the article back again using set() method.
        self.r2.article_set.set([new_article, new_article2])
        self.assertSequenceEqual(self.r.article_set.all(), [self.a])
        self.assertSequenceEqual(
            self.r2.article_set.all(),
            [new_article, new_article2],
        )

        # Because the ForeignKey cannot be null, existing members of the set
        # must remain.
        self.r.article_set.set([new_article])
        self.assertSequenceEqual(
            self.r.article_set.all(),
            [new_article, self.a],
        )
        self.assertSequenceEqual(self.r2.article_set.all(), [new_article2])
        # Reporter cannot be null - there should not be a clear or remove method
        self.assertFalse(hasattr(self.r2.article_set, 'remove'))
        self.assertFalse(hasattr(self.r2.article_set, 'clear'))

    def test_assign_fk_id_value(self):
        parent = Parent.objects.create(name='jeff')
        child1 = Child.objects.create(name='frank', parent=parent)
        child2 = Child.objects.create(name='randy', parent=parent)
        parent.bestchild = child1
        parent.save()
        parent.bestchild_id = child2.pk
        parent.save()
        self.assertEqual(parent.bestchild_id, child2.pk)
        self.assertFalse(Parent.bestchild.is_cached(parent))
        self.assertEqual(parent.bestchild, child2)
        self.assertTrue(Parent.bestchild.is_cached(parent))
        # Reassigning the same value doesn't clear cached instance.
        parent.bestchild_id = child2.pk
        self.assertTrue(Parent.bestchild.is_cached(parent))

    def test_assign_fk_id_none(self):
        parent = Parent.objects.create(name='jeff')
        child = Child.objects.create(name='frank', parent=parent)
        parent.bestchild = child
        parent.save()
        parent.bestchild_id = None
        parent.save()
        self.assertIsNone(parent.bestchild_id)
        self.assertFalse(Parent.bestchild.is_cached(parent))
        self.assertIsNone(parent.bestchild)
        self.assertTrue(Parent.bestchild.is_cached(parent))

    def test_selects(self):
        new_article1 = self.r.article_set.create(
            headline="John's second story",
            pub_date=datetime.date(2005, 7, 29),
        )
        new_article2 = self.r2.article_set.create(
            headline="Paul's story",
            pub_date=datetime.date(2006, 1, 17),
        )
        # Reporter objects have access to their related Article objects.
        self.assertSequenceEqual(
            self.r.article_set.all(),
            [new_article1, self.a],
        )
        self.assertSequenceEqual(self.r.article_set.filter(headline__startswith='This'), [self.a])
        self.assertEqual(self.r.article_set.count(), 2)
        self.assertEqual(self.r2.article_set.count(), 1)
        # Get articles by id
        self.assertSequenceEqual(Article.objects.filter(id__exact=self.a.id), [self.a])
        self.assertSequenceEqual(Article.objects.filter(pk=self.a.id), [self.a])
        # Query on an article property
        self.assertSequenceEqual(Article.objects.filter(headline__startswith='This'), [self.a])
        # The API automatically follows relationships as far as you need.
        # Use double underscores to separate relationships.
        # This works as many levels deep as you want. There's no limit.
        # Find all Articles for any Reporter whose first name is "John".
        self.assertSequenceEqual(
            Article.objects.filter(reporter__first_name__exact='John'),
            [new_article1, self.a],
        )
        # Implied __exact also works
        self.assertSequenceEqual(
            Article.objects.filter(reporter__first_name='John'),
            [new_article1, self.a],
        )
        # Query twice over the related field.
        self.assertSequenceEqual(
            Article.objects.filter(reporter__first_name__exact='John', reporter__last_name__exact='Smith'),
            [new_article1, self.a],
        )
        # The underlying query only makes one join when a related table is referenced twice.
        queryset = Article.objects.filter(reporter__first_name__exact='John', reporter__last_name__exact='Smith')
        self.assertNumQueries(1, list, queryset)
        self.assertEqual(queryset.query.get_compiler(queryset.db).as_sql()[0].count('INNER JOIN'), 1)

        # The automatically joined table has a predictable name.
        self.assertSequenceEqual(
            Article.objects.filter(reporter__first_name__exact='John').extra(
                where=["many_to_one_reporter.last_name='Smith'"]),
            [new_article1, self.a],
        )
        # ... and should work fine with the string that comes out of forms.Form.cleaned_data
        self.assertQuerysetEqual(
            (Article.objects
                .filter(reporter__first_name__exact='John')
                .extra(where=["many_to_one_reporter.last_name='%s'" % 'Smith'])),
            [new_article1, self.a]
        )
        # Find all Articles for a Reporter.
        # Use direct ID check, pk check, and object comparison
        self.assertSequenceEqual(
            Article.objects.filter(reporter__id__exact=self.r.id),
            [new_article1, self.a],
        )
        self.assertSequenceEqual(
            Article.objects.filter(reporter__pk=self.r.id),
            [new_article1, self.a],
        )
        self.assertSequenceEqual(
            Article.objects.filter(reporter=self.r.id),
            [new_article1, self.a],
        )
        self.assertSequenceEqual(
            Article.objects.filter(reporter=self.r),
            [new_article1, self.a],
        )
        self.assertSequenceEqual(
            Article.objects.filter(reporter__in=[self.r.id, self.r2.id]).distinct(),
            [new_article1, new_article2, self.a],
        )
        self.assertSequenceEqual(
            Article.objects.filter(reporter__in=[self.r, self.r2]).distinct(),
            [new_article1, new_article2, self.a],
        )
        # You can also use a queryset instead of a literal list of instances.
        # The queryset must be reduced to a list of values using values(),
        # then converted into a query
        self.assertSequenceEqual(
            Article.objects.filter(
                reporter__in=Reporter.objects.filter(first_name='John').values('pk').query
            ).distinct(),
            [new_article1, self.a],
        )

    def test_reverse_selects(self):
        a3 = Article.objects.create(
            headline="Third article",
            pub_date=datetime.date(2005, 7, 27),
            reporter_id=self.r.id,
        )
        Article.objects.create(
            headline="Fourth article",
            pub_date=datetime.date(2005, 7, 27),
            reporter_id=self.r.id,
        )
        john_smith = [self.r]
        # Reporters can be queried
        self.assertSequenceEqual(Reporter.objects.filter(id__exact=self.r.id), john_smith)
        self.assertSequenceEqual(Reporter.objects.filter(pk=self.r.id), john_smith)
        self.assertSequenceEqual(Reporter.objects.filter(first_name__startswith='John'), john_smith)
        # Reporters can query in opposite direction of ForeignKey definition
        self.assertSequenceEqual(Reporter.objects.filter(article__id__exact=self.a.id), john_smith)
        self.assertSequenceEqual(Reporter.objects.filter(article__pk=self.a.id), john_smith)
        self.assertSequenceEqual(Reporter.objects.filter(article=self.a.id), john_smith)
        self.assertSequenceEqual(Reporter.objects.filter(article=self.a), john_smith)
        self.assertSequenceEqual(Reporter.objects.filter(article__in=[self.a.id, a3.id]).distinct(), john_smith)
        self.assertSequenceEqual(Reporter.objects.filter(article__in=[self.a.id, a3]).distinct(), john_smith)
        self.assertSequenceEqual(Reporter.objects.filter(article__in=[self.a, a3]).distinct(), john_smith)
        self.assertCountEqual(
            Reporter.objects.filter(article__headline__startswith='T'),
            [self.r, self.r],
        )
        self.assertSequenceEqual(Reporter.objects.filter(article__headline__startswith='T').distinct(), john_smith)

        # Counting in the opposite direction works in conjunction with distinct()
        self.assertEqual(Reporter.objects.filter(article__headline__startswith='T').count(), 2)
        self.assertEqual(Reporter.objects.filter(article__headline__startswith='T').distinct().count(), 1)

        # Queries can go round in circles.
        self.assertCountEqual(
            Reporter.objects.filter(article__reporter__first_name__startswith='John'),
            [self.r, self.r, self.r],
        )
        self.assertSequenceEqual(
            Reporter.objects.filter(article__reporter__first_name__startswith='John').distinct(),
            john_smith
        )
        self.assertSequenceEqual(Reporter.objects.filter(article__reporter__exact=self.r).distinct(), john_smith)

        # Implied __exact also works.
        self.assertSequenceEqual(Reporter.objects.filter(article__reporter=self.r).distinct(), john_smith)

        # It's possible to use values() calls across many-to-one relations.
        # (Note, too, that we clear the ordering here so as not to drag the
        # 'headline' field into the columns being used to determine uniqueness)
        d = {'reporter__first_name': 'John', 'reporter__last_name': 'Smith'}
        qs = Article.objects.filter(
            reporter=self.r,
        ).distinct().order_by().values('reporter__first_name', 'reporter__last_name')
        self.assertEqual([d], list(qs))

    def test_select_related(self):
        # Article.objects.select_related().dates() works properly when there
        # are multiple Articles with the same date but different foreign-key
        # objects (Reporters).
        r1 = Reporter.objects.create(first_name='Mike', last_name='Royko', email='royko@suntimes.com')
        r2 = Reporter.objects.create(first_name='John', last_name='Kass', email='jkass@tribune.com')
        Article.objects.create(headline='First', pub_date=datetime.date(1980, 4, 23), reporter=r1)
        Article.objects.create(headline='Second', pub_date=datetime.date(1980, 4, 23), reporter=r2)
        self.assertEqual(
            list(Article.objects.select_related().dates('pub_date', 'day')),
            [datetime.date(1980, 4, 23), datetime.date(2005, 7, 27)]
        )
        self.assertEqual(
            list(Article.objects.select_related().dates('pub_date', 'month')),
            [datetime.date(1980, 4, 1), datetime.date(2005, 7, 1)]
        )
        self.assertEqual(
            list(Article.objects.select_related().dates('pub_date', 'year')),
            [datetime.date(1980, 1, 1), datetime.date(2005, 1, 1)]
        )

    def test_delete(self):
        new_article1 = self.r.article_set.create(
            headline="John's second story",
            pub_date=datetime.date(2005, 7, 29),
        )
        new_article2 = self.r2.article_set.create(
            headline="Paul's story",
            pub_date=datetime.date(2006, 1, 17),
        )
        new_article3 = Article.objects.create(
            headline="Third article",
            pub_date=datetime.date(2005, 7, 27),
            reporter_id=self.r.id,
        )
        new_article4 = Article.objects.create(
            headline="Fourth article",
            pub_date=datetime.date(2005, 7, 27),
            reporter_id=str(self.r.id),
        )
        # If you delete a reporter, their articles will be deleted.
        self.assertSequenceEqual(
            Article.objects.all(),
            [new_article4, new_article1, new_article2, new_article3, self.a],
        )
        self.assertSequenceEqual(
            Reporter.objects.order_by('first_name'),
            [self.r, self.r2],
        )
        self.r2.delete()
        self.assertSequenceEqual(
            Article.objects.all(),
            [new_article4, new_article1, new_article3, self.a],
        )
        self.assertSequenceEqual(Reporter.objects.order_by('first_name'), [self.r])
        # You can delete using a JOIN in the query.
        Reporter.objects.filter(article__headline__startswith='This').delete()
        self.assertSequenceEqual(Reporter.objects.all(), [])
        self.assertSequenceEqual(Article.objects.all(), [])

    def test_explicit_fk(self):
        # Create a new Article with get_or_create using an explicit value
        # for a ForeignKey.
        a2, created = Article.objects.get_or_create(
            headline="John's second test",
            pub_date=datetime.date(2011, 5, 7),
            reporter_id=self.r.id,
        )
        self.assertTrue(created)
        self.assertEqual(a2.reporter.id, self.r.id)

        # You can specify filters containing the explicit FK value.
        self.assertSequenceEqual(
            Article.objects.filter(reporter_id__exact=self.r.id),
            [a2, self.a],
        )

        # Create an Article by Paul for the same date.
        a3 = Article.objects.create(
            headline="Paul's commentary",
            pub_date=datetime.date(2011, 5, 7),
            reporter_id=self.r2.id,
        )
        self.assertEqual(a3.reporter.id, self.r2.id)

        # Get should respect explicit foreign keys as well.
        msg = 'get() returned more than one Article -- it returned 2!'
        with self.assertRaisesMessage(MultipleObjectsReturned, msg):
            Article.objects.get(reporter_id=self.r.id)
        self.assertEqual(
            repr(a3),
            repr(Article.objects.get(reporter_id=self.r2.id, pub_date=datetime.date(2011, 5, 7)))
        )

    def test_deepcopy_and_circular_references(self):
        # Regression for #12876 -- Model methods that include queries that
        # recursive don't cause recursion depth problems under deepcopy.
        self.r.cached_query = Article.objects.filter(reporter=self.r)
        self.assertEqual(repr(deepcopy(self.r)), "<Reporter: John Smith>")

    def test_manager_class_caching(self):
        r1 = Reporter.objects.create(first_name='Mike')
        r2 = Reporter.objects.create(first_name='John')

        # Same twice
        self.assertIs(r1.article_set.__class__, r1.article_set.__class__)

        # Same as each other
        self.assertIs(r1.article_set.__class__, r2.article_set.__class__)

    def test_create_relation_with_gettext_lazy(self):
        reporter = Reporter.objects.create(first_name='John', last_name='Smith', email='john.smith@example.com')
        lazy = gettext_lazy('test')
        reporter.article_set.create(headline=lazy, pub_date=datetime.date(2011, 6, 10))
        notlazy = str(lazy)
        article = reporter.article_set.get()
        self.assertEqual(article.headline, notlazy)

    def test_values_list_exception(self):
        expected_message = "Cannot resolve keyword 'notafield' into field. Choices are: %s"
        reporter_fields = ', '.join(sorted(f.name for f in Reporter._meta.get_fields()))
        with self.assertRaisesMessage(FieldError, expected_message % reporter_fields):
            Article.objects.values_list('reporter__notafield')
        article_fields = ', '.join(['EXTRA'] + sorted(f.name for f in Article._meta.get_fields()))
        with self.assertRaisesMessage(FieldError, expected_message % article_fields):
            Article.objects.extra(select={'EXTRA': 'EXTRA_SELECT'}).values_list('notafield')

    def test_fk_assignment_and_related_object_cache(self):
        # Tests of ForeignKey assignment and the related-object cache (see #6886).

        p = Parent.objects.create(name="Parent")
        c = Child.objects.create(name="Child", parent=p)

        # Look up the object again so that we get a "fresh" object.
        c = Child.objects.get(name="Child")
        p = c.parent

        # Accessing the related object again returns the exactly same object.
        self.assertIs(c.parent, p)

        # But if we kill the cache, we get a new object.
        del c._state.fields_cache['parent']
        self.assertIsNot(c.parent, p)

        # Assigning a new object results in that object getting cached immediately.
        p2 = Parent.objects.create(name="Parent 2")
        c.parent = p2
        self.assertIs(c.parent, p2)

        # Assigning None succeeds if field is null=True.
        p.bestchild = None
        self.assertIsNone(p.bestchild)

        # bestchild should still be None after saving.
        p.save()
        self.assertIsNone(p.bestchild)

        # bestchild should still be None after fetching the object again.
        p = Parent.objects.get(name="Parent")
        self.assertIsNone(p.bestchild)

        # Assigning None will not fail: Child.parent is null=False.
        setattr(c, "parent", None)

        # You also can't assign an object of the wrong type here
        msg = (
            'Cannot assign "<First: First object (1)>": "Child.parent" must '
            'be a "Parent" instance.'
        )
        with self.assertRaisesMessage(ValueError, msg):
            setattr(c, "parent", First(id=1, second=1))

        # You can assign None to Child.parent during object creation.
        Child(name='xyzzy', parent=None)

        # But when trying to save a Child with parent=None, the database will
        # raise IntegrityError.
        with self.assertRaises(IntegrityError), transaction.atomic():
            Child.objects.create(name='xyzzy', parent=None)

        # Creation using keyword argument should cache the related object.
        p = Parent.objects.get(name="Parent")
        c = Child(parent=p)
        self.assertIs(c.parent, p)

        # Creation using keyword argument and unsaved related instance (#8070).
        p = Parent()
        msg = "save() prohibited to prevent data loss due to unsaved related object 'parent'."
        with self.assertRaisesMessage(ValueError, msg):
            Child.objects.create(parent=p)

        msg = "save() prohibited to prevent data loss due to unsaved related object 'parent'."
        with self.assertRaisesMessage(ValueError, msg):
            ToFieldChild.objects.create(parent=p)

        # Creation using attname keyword argument and an id will cause the
        # related object to be fetched.
        p = Parent.objects.get(name="Parent")
        c = Child(parent_id=p.id)
        self.assertIsNot(c.parent, p)
        self.assertEqual(c.parent, p)

    def test_save_nullable_fk_after_parent(self):
        parent = Parent()
        child = ChildNullableParent(parent=parent)
        parent.save()
        child.save()
        child.refresh_from_db()
        self.assertEqual(child.parent, parent)

    def test_save_nullable_fk_after_parent_with_to_field(self):
        parent = Parent(name='jeff')
        child = ToFieldChild(parent=parent)
        parent.save()
        child.save()
        child.refresh_from_db()
        self.assertEqual(child.parent, parent)
        self.assertEqual(child.parent_id, parent.name)

    def test_fk_to_bigautofield(self):
        ch = City.objects.create(name='Chicago')
        District.objects.create(city=ch, name='Far South')
        District.objects.create(city=ch, name='North')

        ny = City.objects.create(name='New York', id=2 ** 33)
        District.objects.create(city=ny, name='Brooklyn')
        District.objects.create(city=ny, name='Manhattan')

    def test_fk_to_smallautofield(self):
        us = Country.objects.create(name='United States')
        City.objects.create(country=us, name='Chicago')
        City.objects.create(country=us, name='New York')

        uk = Country.objects.create(name='United Kingdom', id=2 ** 11)
        City.objects.create(country=uk, name='London')
        City.objects.create(country=uk, name='Edinburgh')

    def test_multiple_foreignkeys(self):
        # Test of multiple ForeignKeys to the same model (bug #7125).
        c1 = Category.objects.create(name='First')
        c2 = Category.objects.create(name='Second')
        c3 = Category.objects.create(name='Third')
        r1 = Record.objects.create(category=c1)
        r2 = Record.objects.create(category=c1)
        r3 = Record.objects.create(category=c2)
        r4 = Record.objects.create(category=c2)
        r5 = Record.objects.create(category=c3)
        Relation.objects.create(left=r1, right=r2)
        Relation.objects.create(left=r3, right=r4)
        rel = Relation.objects.create(left=r1, right=r3)
        Relation.objects.create(left=r5, right=r2)
        Relation.objects.create(left=r3, right=r2)

        q1 = Relation.objects.filter(left__category__name__in=['First'], right__category__name__in=['Second'])
        self.assertSequenceEqual(q1, [rel])

        q2 = Category.objects.filter(record__left_set__right__category__name='Second').order_by('name')
        self.assertSequenceEqual(q2, [c1, c2])

        p = Parent.objects.create(name="Parent")
        c = Child.objects.create(name="Child", parent=p)
        msg = 'Cannot assign "%r": "Child.parent" must be a "Parent" instance.' % c
        with self.assertRaisesMessage(ValueError, msg):
            Child.objects.create(name="Grandchild", parent=c)

    def test_fk_instantiation_outside_model(self):
        # Regression for #12190 -- Should be able to instantiate a FK outside
        # of a model, and interrogate its related field.
        cat = models.ForeignKey(Category, models.CASCADE)
        self.assertEqual('id', cat.remote_field.get_related_field().name)

    def test_relation_unsaved(self):
        # The <field>_set manager does not join on Null value fields (#17541)
        Third.objects.create(name='Third 1')
        Third.objects.create(name='Third 2')
        th = Third(name="testing")
        # The object isn't saved and thus the relation field is null - we won't even
        # execute a query in this case.
        with self.assertNumQueries(0):
            self.assertEqual(th.child_set.count(), 0)
        th.save()
        # Now the model is saved, so we will need to execute a query.
        with self.assertNumQueries(1):
            self.assertEqual(th.child_set.count(), 0)

    def test_related_object(self):
        public_school = School.objects.create(is_public=True)
        public_student = Student.objects.create(school=public_school)

        private_school = School.objects.create(is_public=False)
        private_student = Student.objects.create(school=private_school)

        # Only one school is available via all() due to the custom default manager.
        self.assertSequenceEqual(School.objects.all(), [public_school])

        self.assertEqual(public_student.school, public_school)

        # Make sure the base manager is used so that a student can still access
        # its related school even if the default manager doesn't normally
        # allow it.
        self.assertEqual(private_student.school, private_school)

        School._meta.base_manager_name = 'objects'
        School._meta._expire_cache()
        try:
            private_student = Student.objects.get(pk=private_student.pk)
            with self.assertRaises(School.DoesNotExist):
                private_student.school
        finally:
            School._meta.base_manager_name = None
            School._meta._expire_cache()

    def test_hasattr_related_object(self):
        # The exception raised on attribute access when a related object
        # doesn't exist should be an instance of a subclass of `AttributeError`
        # refs #21563
        self.assertFalse(hasattr(Article(), 'reporter'))

    def test_clear_after_prefetch(self):
        c = City.objects.create(name='Musical City')
        d = District.objects.create(name='Ladida', city=c)
        city = City.objects.prefetch_related('districts').get(id=c.id)
        self.assertSequenceEqual(city.districts.all(), [d])
        city.districts.clear()
        self.assertSequenceEqual(city.districts.all(), [])

    def test_remove_after_prefetch(self):
        c = City.objects.create(name='Musical City')
        d = District.objects.create(name='Ladida', city=c)
        city = City.objects.prefetch_related('districts').get(id=c.id)
        self.assertSequenceEqual(city.districts.all(), [d])
        city.districts.remove(d)
        self.assertSequenceEqual(city.districts.all(), [])

    def test_add_after_prefetch(self):
        c = City.objects.create(name='Musical City')
        District.objects.create(name='Ladida', city=c)
        d2 = District.objects.create(name='Ladidu')
        city = City.objects.prefetch_related('districts').get(id=c.id)
        self.assertEqual(city.districts.count(), 1)
        city.districts.add(d2)
        self.assertEqual(city.districts.count(), 2)

    def test_set_after_prefetch(self):
        c = City.objects.create(name='Musical City')
        District.objects.create(name='Ladida', city=c)
        d2 = District.objects.create(name='Ladidu')
        city = City.objects.prefetch_related('districts').get(id=c.id)
        self.assertEqual(city.districts.count(), 1)
        city.districts.set([d2])
        self.assertSequenceEqual(city.districts.all(), [d2])

    def test_add_then_remove_after_prefetch(self):
        c = City.objects.create(name='Musical City')
        District.objects.create(name='Ladida', city=c)
        d2 = District.objects.create(name='Ladidu')
        city = City.objects.prefetch_related('districts').get(id=c.id)
        self.assertEqual(city.districts.count(), 1)
        city.districts.add(d2)
        self.assertEqual(city.districts.count(), 2)
        city.districts.remove(d2)
        self.assertEqual(city.districts.count(), 1)

    def test_cached_relation_invalidated_on_save(self):
        """
        Model.save() invalidates stale ForeignKey relations after a primary key
        assignment.
        """
        self.assertEqual(self.a.reporter, self.r)  # caches a.reporter
        self.a.reporter_id = self.r2.pk
        self.a.save()
        self.assertEqual(self.a.reporter, self.r2)

    def test_cached_foreign_key_with_to_field_not_cleared_by_save(self):
        parent = Parent.objects.create(name='a')
        child = ToFieldChild.objects.create(parent=parent)
        with self.assertNumQueries(0):
            self.assertIs(child.parent, parent)

    def test_reverse_foreign_key_instance_to_field_caching(self):
        parent = Parent.objects.create(name='a')
        ToFieldChild.objects.create(parent=parent)
        child = parent.to_field_children.get()
        with self.assertNumQueries(0):
            self.assertIs(child.parent, parent)

    def test_add_remove_set_by_pk_raises(self):
        usa = Country.objects.create(name='United States')
        chicago = City.objects.create(name='Chicago')
        msg = "'City' instance expected, got %s" % chicago.pk
        with self.assertRaisesMessage(TypeError, msg):
            usa.cities.add(chicago.pk)
        with self.assertRaisesMessage(TypeError, msg):
            usa.cities.remove(chicago.pk)
        with self.assertRaisesMessage(TypeError, msg):
            usa.cities.set([chicago.pk])