Viewing github.com/django/django-stable-4.1.x/tests/many_to

```
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,
```
```
    ChildStringPrimaryKeyParent,
```
```
    City,
```
```
    Country,
```
```
    District,
```
```
    First,
```
```
    Parent,
```
```
    ParentStringPrimaryKey,
```
```
    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="[email protected]")

```
        cls.r.save()
```
```
        cls.r2 = Reporter(
```

            first_name="Paul", last_name="Jones", email="[email protected]"

```
        )
```
```
        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="[email protected]"

```
        )
```
```
        r2 = Reporter.objects.create(
```

            first_name="John", last_name="Kass", email="[email protected]"

```
        )
```
```
        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="[email protected]"

```
        )
```
```
        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)

        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_save_fk_after_parent_with_non_numeric_pk_set_on_child(self):

        parent = ParentStringPrimaryKey()

        child = ChildStringPrimaryKeyParent(parent=parent)

```
        child.parent.name = "jeff"
```
```
        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):
```

        Third.objects.create(name="Third 1")

        Third.objects.create(name="Third 2")

```
        th = Third(name="testing")
```

        # The object isn't saved and the relation cannot be used.

```
        msg = (
```

            "'Third' instance needs to have a primary key value before this "

            "relationship can be used."

```
        )
```

        with self.assertRaisesMessage(ValueError, msg):

```
            th.child_set.count()
```

        # The reverse foreign key manager can be created.

        self.assertEqual(th.child_set.model, Third)

```
        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])