django/tests/foreign_object/tests.py

import copy
import datetime
import pickle
from operator import attrgetter

from django.core.exceptions import FieldError
from django.db import connection, models
from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
from django.test.utils import isolate_apps
from django.utils import translation

from .models import (
    Article,
    ArticleIdea,
    ArticleTag,
    ArticleTranslation,
    Country,
    Friendship,
    Group,
    Membership,
    NewsArticle,
    Person,
)

# Note that these tests are testing internal implementation details.
# ForeignObject is not part of public API.


class MultiColumnFKTests(TestCase):
    @classmethod
    def setUpTestData(cls):
        # Creating countries
        cls.usa = Country.objects.create(name="United States of America")
        cls.soviet_union = Country.objects.create(name="Soviet Union")
        # Creating People
        cls.bob = Person.objects.create(name="Bob", person_country=cls.usa)
        cls.jim = Person.objects.create(name="Jim", person_country=cls.usa)
        cls.george = Person.objects.create(name="George", person_country=cls.usa)

        cls.jane = Person.objects.create(name="Jane", person_country=cls.soviet_union)
        cls.mark = Person.objects.create(name="Mark", person_country=cls.soviet_union)
        cls.sam = Person.objects.create(name="Sam", person_country=cls.soviet_union)

        # Creating Groups
        cls.kgb = Group.objects.create(name="KGB", group_country=cls.soviet_union)
        cls.cia = Group.objects.create(name="CIA", group_country=cls.usa)
        cls.republican = Group.objects.create(name="Republican", group_country=cls.usa)
        cls.democrat = Group.objects.create(name="Democrat", group_country=cls.usa)

    def test_get_succeeds_on_multicolumn_match(self):
        # Membership objects have access to their related Person if both
        # country_ids match between them
        membership = Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.bob.id,
            group_id=self.cia.id,
        )

        person = membership.person
        self.assertEqual((person.id, person.name), (self.bob.id, "Bob"))

    def test_get_fails_on_multicolumn_mismatch(self):
        # Membership objects returns DoesNotExist error when there is no
        # Person with the same id and country_id
        membership = Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.jane.id,
            group_id=self.cia.id,
        )

        with self.assertRaises(Person.DoesNotExist):
            getattr(membership, "person")

    def test_reverse_query_returns_correct_result(self):
        # Creating a valid membership because it has the same country has the person
        Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.bob.id,
            group_id=self.cia.id,
        )

        # Creating an invalid membership because it has a different country has
        # the person.
        Membership.objects.create(
            membership_country_id=self.soviet_union.id,
            person_id=self.bob.id,
            group_id=self.republican.id,
        )

        with self.assertNumQueries(1):
            membership = self.bob.membership_set.get()
            self.assertEqual(membership.group_id, self.cia.id)
            self.assertIs(membership.person, self.bob)

    def test_query_filters_correctly(self):
        # Creating a to valid memberships
        Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.bob.id,
            group_id=self.cia.id,
        )
        Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.jim.id,
            group_id=self.cia.id,
        )

        # Creating an invalid membership
        Membership.objects.create(
            membership_country_id=self.soviet_union.id,
            person_id=self.george.id,
            group_id=self.cia.id,
        )

        self.assertQuerySetEqual(
            Membership.objects.filter(person__name__contains="o"),
            [self.bob.id],
            attrgetter("person_id"),
        )

    def test_reverse_query_filters_correctly(self):
        timemark = datetime.datetime.now(tz=datetime.UTC).replace(tzinfo=None)
        timedelta = datetime.timedelta(days=1)

        # Creating a to valid memberships
        Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.bob.id,
            group_id=self.cia.id,
            date_joined=timemark - timedelta,
        )
        Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.jim.id,
            group_id=self.cia.id,
            date_joined=timemark + timedelta,
        )

        # Creating an invalid membership
        Membership.objects.create(
            membership_country_id=self.soviet_union.id,
            person_id=self.george.id,
            group_id=self.cia.id,
            date_joined=timemark + timedelta,
        )

        self.assertQuerySetEqual(
            Person.objects.filter(membership__date_joined__gte=timemark),
            ["Jim"],
            attrgetter("name"),
        )

    def test_forward_in_lookup_filters_correctly(self):
        Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.bob.id,
            group_id=self.cia.id,
        )
        Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.jim.id,
            group_id=self.cia.id,
        )

        # Creating an invalid membership
        Membership.objects.create(
            membership_country_id=self.soviet_union.id,
            person_id=self.george.id,
            group_id=self.cia.id,
        )

        self.assertQuerySetEqual(
            Membership.objects.filter(person__in=[self.george, self.jim]),
            [
                self.jim.id,
            ],
            attrgetter("person_id"),
        )
        self.assertQuerySetEqual(
            Membership.objects.filter(person__in=Person.objects.filter(name="Jim")),
            [
                self.jim.id,
            ],
            attrgetter("person_id"),
        )

    def test_double_nested_query(self):
        m1 = Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.bob.id,
            group_id=self.cia.id,
        )
        m2 = Membership.objects.create(
            membership_country_id=self.usa.id,
            person_id=self.jim.id,
            group_id=self.cia.id,
        )
        Friendship.objects.create(
            from_friend_country_id=self.usa.id,
            from_friend_id=self.bob.id,
            to_friend_country_id=self.usa.id,
            to_friend_id=self.jim.id,
        )
        self.assertSequenceEqual(
            Membership.objects.filter(
                person__in=Person.objects.filter(
                    from_friend__in=Friendship.objects.filter(
                        to_friend__in=Person.objects.all()
                    )
                )
            ),
            [m1],
        )
        self.assertSequenceEqual(
            Membership.objects.exclude(
                person__in=Person.objects.filter(
                    from_friend__in=Friendship.objects.filter(
                        to_friend__in=Person.objects.all()
                    )
                )
            ),
            [m2],
        )

    def test_query_does_not_mutate(self):
        """
        Recompiling the same subquery doesn't mutate it.
        """
        queryset = Friendship.objects.filter(to_friend__in=Person.objects.all())
        self.assertEqual(str(queryset.query), str(queryset.query))

    def test_select_related_foreignkey_forward_works(self):
        Membership.objects.create(
            membership_country=self.usa, person=self.bob, group=self.cia
        )
        Membership.objects.create(
            membership_country=self.usa, person=self.jim, group=self.democrat
        )

        with self.assertNumQueries(1):
            people = [
                m.person
                for m in Membership.objects.select_related("person").order_by("pk")
            ]

        normal_people = [m.person for m in Membership.objects.order_by("pk")]
        self.assertEqual(people, normal_people)

    def test_prefetch_foreignobject_forward(self):
        Membership.objects.create(
            membership_country=self.usa, person=self.bob, group=self.cia
        )
        Membership.objects.create(
            membership_country=self.usa, person=self.jim, group=self.democrat
        )

        with self.assertNumQueries(2):
            people = [
                m.person
                for m in Membership.objects.prefetch_related("person").order_by("pk")
            ]

        normal_people = [m.person for m in Membership.objects.order_by("pk")]
        self.assertEqual(people, normal_people)

    def test_prefetch_foreignobject_hidden_forward(self):
        Friendship.objects.create(
            from_friend_country=self.usa,
            from_friend_id=self.bob.id,
            to_friend_country_id=self.usa.id,
            to_friend_id=self.george.id,
        )
        Friendship.objects.create(
            from_friend_country=self.usa,
            from_friend_id=self.bob.id,
            to_friend_country_id=self.soviet_union.id,
            to_friend_id=self.sam.id,
        )
        with self.assertNumQueries(2) as ctx:
            friendships = list(
                Friendship.objects.prefetch_related("to_friend").order_by("pk")
            )
        prefetch_sql = ctx[-1]["sql"]
        # Prefetch queryset should be filtered by all foreign related fields
        # to prevent extra rows from being eagerly fetched.
        prefetch_where_sql = prefetch_sql.split("WHERE")[-1]
        for to_field_name in Friendship.to_friend.field.to_fields:
            to_field = Person._meta.get_field(to_field_name)
            with self.subTest(to_field=to_field):
                self.assertIn(
                    connection.ops.quote_name(to_field.column),
                    prefetch_where_sql,
                )
        self.assertNotIn(" JOIN ", prefetch_sql)
        with self.assertNumQueries(0):
            self.assertEqual(friendships[0].to_friend, self.george)
            self.assertEqual(friendships[1].to_friend, self.sam)

    def test_prefetch_foreignobject_reverse(self):
        Membership.objects.create(
            membership_country=self.usa, person=self.bob, group=self.cia
        )
        Membership.objects.create(
            membership_country=self.usa, person=self.jim, group=self.democrat
        )
        with self.assertNumQueries(2):
            membership_sets = [
                list(p.membership_set.all())
                for p in Person.objects.prefetch_related("membership_set").order_by(
                    "pk"
                )
            ]

        with self.assertNumQueries(7):
            normal_membership_sets = [
                list(p.membership_set.all()) for p in Person.objects.order_by("pk")
            ]
        self.assertEqual(membership_sets, normal_membership_sets)

    def test_m2m_through_forward_returns_valid_members(self):
        # We start out by making sure that the Group 'CIA' has no members.
        self.assertQuerySetEqual(self.cia.members.all(), [])

        Membership.objects.create(
            membership_country=self.usa, person=self.bob, group=self.cia
        )
        Membership.objects.create(
            membership_country=self.usa, person=self.jim, group=self.cia
        )

        # Bob and Jim should be members of the CIA.

        self.assertQuerySetEqual(
            self.cia.members.all(), ["Bob", "Jim"], attrgetter("name")
        )

    def test_m2m_through_reverse_returns_valid_members(self):
        # We start out by making sure that Bob is in no groups.
        self.assertQuerySetEqual(self.bob.groups.all(), [])

        Membership.objects.create(
            membership_country=self.usa, person=self.bob, group=self.cia
        )
        Membership.objects.create(
            membership_country=self.usa, person=self.bob, group=self.republican
        )

        # Bob should be in the CIA and a Republican
        self.assertQuerySetEqual(
            self.bob.groups.all(), ["CIA", "Republican"], attrgetter("name")
        )

    def test_m2m_through_forward_ignores_invalid_members(self):
        # We start out by making sure that the Group 'CIA' has no members.
        self.assertQuerySetEqual(self.cia.members.all(), [])

        # Something adds jane to group CIA but Jane is in Soviet Union which
        # isn't CIA's country.
        Membership.objects.create(
            membership_country=self.usa, person=self.jane, group=self.cia
        )

        # There should still be no members in CIA
        self.assertQuerySetEqual(self.cia.members.all(), [])

    def test_m2m_through_reverse_ignores_invalid_members(self):
        # We start out by making sure that Jane has no groups.
        self.assertQuerySetEqual(self.jane.groups.all(), [])

        # Something adds jane to group CIA but Jane is in Soviet Union which
        # isn't CIA's country.
        Membership.objects.create(
            membership_country=self.usa, person=self.jane, group=self.cia
        )

        # Jane should still not be in any groups
        self.assertQuerySetEqual(self.jane.groups.all(), [])

    def test_m2m_through_on_self_works(self):
        self.assertQuerySetEqual(self.jane.friends.all(), [])

        Friendship.objects.create(
            from_friend_country=self.jane.person_country,
            from_friend=self.jane,
            to_friend_country=self.george.person_country,
            to_friend=self.george,
        )

        self.assertQuerySetEqual(
            self.jane.friends.all(), ["George"], attrgetter("name")
        )

    def test_m2m_through_on_self_ignores_mismatch_columns(self):
        self.assertQuerySetEqual(self.jane.friends.all(), [])

        # Note that we use ids instead of instances. This is because instances
        # on ForeignObject properties will set all related field off of the
        # given instance.
        Friendship.objects.create(
            from_friend_id=self.jane.id,
            to_friend_id=self.george.id,
            to_friend_country_id=self.jane.person_country_id,
            from_friend_country_id=self.george.person_country_id,
        )

        self.assertQuerySetEqual(self.jane.friends.all(), [])

    def test_prefetch_related_m2m_forward_works(self):
        Membership.objects.create(
            membership_country=self.usa, person=self.bob, group=self.cia
        )
        Membership.objects.create(
            membership_country=self.usa, person=self.jim, group=self.democrat
        )

        with self.assertNumQueries(2):
            members_lists = [
                list(g.members.all()) for g in Group.objects.prefetch_related("members")
            ]

        normal_members_lists = [list(g.members.all()) for g in Group.objects.all()]
        self.assertEqual(members_lists, normal_members_lists)

    def test_prefetch_related_m2m_reverse_works(self):
        Membership.objects.create(
            membership_country=self.usa, person=self.bob, group=self.cia
        )
        Membership.objects.create(
            membership_country=self.usa, person=self.jim, group=self.democrat
        )

        with self.assertNumQueries(2):
            groups_lists = [
                list(p.groups.all()) for p in Person.objects.prefetch_related("groups")
            ]

        normal_groups_lists = [list(p.groups.all()) for p in Person.objects.all()]
        self.assertEqual(groups_lists, normal_groups_lists)

    @translation.override("fi")
    def test_translations(self):
        a1 = Article.objects.create(pub_date=datetime.date.today())
        at1_fi = ArticleTranslation(
            article=a1, lang="fi", title="Otsikko", body="Diipadaapa"
        )
        at1_fi.save()
        at2_en = ArticleTranslation(
            article=a1, lang="en", title="Title", body="Lalalalala"
        )
        at2_en.save()

        self.assertEqual(Article.objects.get(pk=a1.pk).active_translation, at1_fi)

        with self.assertNumQueries(1):
            fetched = Article.objects.select_related("active_translation").get(
                active_translation__title="Otsikko"
            )
            self.assertEqual(fetched.active_translation.title, "Otsikko")
        a2 = Article.objects.create(pub_date=datetime.date.today())
        at2_fi = ArticleTranslation(
            article=a2, lang="fi", title="Atsikko", body="Diipadaapa", abstract="dipad"
        )
        at2_fi.save()
        a3 = Article.objects.create(pub_date=datetime.date.today())
        at3_en = ArticleTranslation(
            article=a3, lang="en", title="A title", body="lalalalala", abstract="lala"
        )
        at3_en.save()
        # Test model initialization with active_translation field.
        a3 = Article(id=a3.id, pub_date=a3.pub_date, active_translation=at3_en)
        a3.save()
        self.assertEqual(
            list(Article.objects.filter(active_translation__abstract=None)), [a1, a3]
        )
        self.assertEqual(
            list(
                Article.objects.filter(
                    active_translation__abstract=None,
                    active_translation__pk__isnull=False,
                )
            ),
            [a1],
        )

        with translation.override("en"):
            self.assertEqual(
                list(Article.objects.filter(active_translation__abstract=None)),
                [a1, a2],
            )

    def test_foreign_key_raises_informative_does_not_exist(self):
        referrer = ArticleTranslation()
        with self.assertRaisesMessage(
            Article.DoesNotExist, "ArticleTranslation has no article"
        ):
            referrer.article

    def test_foreign_key_related_query_name(self):
        a1 = Article.objects.create(pub_date=datetime.date.today())
        ArticleTag.objects.create(article=a1, name="foo")
        self.assertEqual(Article.objects.filter(tag__name="foo").count(), 1)
        self.assertEqual(Article.objects.filter(tag__name="bar").count(), 0)
        msg = (
            "Cannot resolve keyword 'tags' into field. Choices are: "
            "active_translation, active_translation_q, articletranslation, "
            "id, idea_things, newsarticle, pub_date, tag"
        )
        with self.assertRaisesMessage(FieldError, msg):
            Article.objects.filter(tags__name="foo")

    def test_many_to_many_related_query_name(self):
        a1 = Article.objects.create(pub_date=datetime.date.today())
        i1 = ArticleIdea.objects.create(name="idea1")
        a1.ideas.add(i1)
        self.assertEqual(Article.objects.filter(idea_things__name="idea1").count(), 1)
        self.assertEqual(Article.objects.filter(idea_things__name="idea2").count(), 0)
        msg = (
            "Cannot resolve keyword 'ideas' into field. Choices are: "
            "active_translation, active_translation_q, articletranslation, "
            "id, idea_things, newsarticle, pub_date, tag"
        )
        with self.assertRaisesMessage(FieldError, msg):
            Article.objects.filter(ideas__name="idea1")

    @translation.override("fi")
    def test_inheritance(self):
        na = NewsArticle.objects.create(pub_date=datetime.date.today())
        ArticleTranslation.objects.create(
            article=na, lang="fi", title="foo", body="bar"
        )
        self.assertSequenceEqual(
            NewsArticle.objects.select_related("active_translation"), [na]
        )
        with self.assertNumQueries(1):
            self.assertEqual(
                NewsArticle.objects.select_related("active_translation")[
                    0
                ].active_translation.title,
                "foo",
            )

    @skipUnlessDBFeature("has_bulk_insert")
    def test_batch_create_foreign_object(self):
        objs = [
            Person(name="abcd_%s" % i, person_country=self.usa) for i in range(0, 5)
        ]
        Person.objects.bulk_create(objs, 10)

    def test_isnull_lookup(self):
        m1 = Membership.objects.create(
            person_id=self.bob.id,
            membership_country_id=self.usa.id,
            group_id=None,
        )
        m2 = Membership.objects.create(
            person_id=self.jim.id,
            membership_country_id=None,
            group_id=self.cia.id,
        )
        m3 = Membership.objects.create(
            person_id=self.jane.id,
            membership_country_id=None,
            group_id=None,
        )
        m4 = Membership.objects.create(
            person_id=self.george.id,
            membership_country_id=self.soviet_union.id,
            group_id=self.kgb.id,
        )
        for member in [m1, m2, m3]:
            with self.assertRaises(Membership.group.RelatedObjectDoesNotExist):
                getattr(member, "group")
        self.assertSequenceEqual(
            Membership.objects.filter(group__isnull=True),
            [m1, m2, m3],
        )
        self.assertSequenceEqual(
            Membership.objects.filter(group__isnull=False),
            [m4],
        )


class TestModelCheckTests(SimpleTestCase):
    @isolate_apps("foreign_object")
    def test_check_composite_foreign_object(self):
        class Parent(models.Model):
            a = models.PositiveIntegerField()
            b = models.PositiveIntegerField()

            class Meta:
                unique_together = (("a", "b"),)

        class Child(models.Model):
            a = models.PositiveIntegerField()
            b = models.PositiveIntegerField()
            value = models.CharField(max_length=255)
            parent = models.ForeignObject(
                Parent,
                on_delete=models.SET_NULL,
                from_fields=("a", "b"),
                to_fields=("a", "b"),
                related_name="children",
            )

        self.assertEqual(Child._meta.get_field("parent").check(from_model=Child), [])

    @isolate_apps("foreign_object")
    def test_check_subset_composite_foreign_object(self):
        class Parent(models.Model):
            a = models.PositiveIntegerField()
            b = models.PositiveIntegerField()
            c = models.PositiveIntegerField()

            class Meta:
                unique_together = (("a", "b"),)

        class Child(models.Model):
            a = models.PositiveIntegerField()
            b = models.PositiveIntegerField()
            c = models.PositiveIntegerField()
            d = models.CharField(max_length=255)
            parent = models.ForeignObject(
                Parent,
                on_delete=models.SET_NULL,
                from_fields=("a", "b", "c"),
                to_fields=("a", "b", "c"),
                related_name="children",
            )

        self.assertEqual(Child._meta.get_field("parent").check(from_model=Child), [])


class TestExtraJoinFilterQ(TestCase):
    @translation.override("fi")
    def test_extra_join_filter_q(self):
        a = Article.objects.create(pub_date=datetime.datetime.today())
        ArticleTranslation.objects.create(
            article=a, lang="fi", title="title", body="body"
        )
        qs = Article.objects.all()
        with self.assertNumQueries(2):
            self.assertEqual(qs[0].active_translation_q.title, "title")
        qs = qs.select_related("active_translation_q")
        with self.assertNumQueries(1):
            self.assertEqual(qs[0].active_translation_q.title, "title")


class TestCachedPathInfo(TestCase):
    def test_equality(self):
        """
        The path_infos and reverse_path_infos attributes are equivalent to
        calling the get_<method>() with no arguments.
        """
        foreign_object = Membership._meta.get_field("person")
        self.assertEqual(
            foreign_object.path_infos,
            foreign_object.get_path_info(),
        )
        self.assertEqual(
            foreign_object.reverse_path_infos,
            foreign_object.get_reverse_path_info(),
        )

    def test_copy_removes_direct_cached_values(self):
        """
        Shallow copying a ForeignObject (or a ForeignObjectRel) removes the
        object's direct cached PathInfo values.
        """
        foreign_object = Membership._meta.get_field("person")
        # Trigger storage of cached_property into ForeignObject's __dict__.
        foreign_object.path_infos
        foreign_object.reverse_path_infos
        # The ForeignObjectRel doesn't have reverse_path_infos.
        foreign_object.remote_field.path_infos
        self.assertIn("path_infos", foreign_object.__dict__)
        self.assertIn("reverse_path_infos", foreign_object.__dict__)
        self.assertIn("path_infos", foreign_object.remote_field.__dict__)
        # Cached value is removed via __getstate__() on ForeignObjectRel
        # because no __copy__() method exists, so __reduce_ex__() is used.
        remote_field_copy = copy.copy(foreign_object.remote_field)
        self.assertNotIn("path_infos", remote_field_copy.__dict__)
        # Cached values are removed via __copy__() on ForeignObject for
        # consistency of behavior.
        foreign_object_copy = copy.copy(foreign_object)
        self.assertNotIn("path_infos", foreign_object_copy.__dict__)
        self.assertNotIn("reverse_path_infos", foreign_object_copy.__dict__)
        # ForeignObjectRel's remains because it's part of a shallow copy.
        self.assertIn("path_infos", foreign_object_copy.remote_field.__dict__)

    def test_deepcopy_removes_cached_values(self):
        """
        Deep copying a ForeignObject removes the object's cached PathInfo
        values, including those of the related ForeignObjectRel.
        """
        foreign_object = Membership._meta.get_field("person")
        # Trigger storage of cached_property into ForeignObject's __dict__.
        foreign_object.path_infos
        foreign_object.reverse_path_infos
        # The ForeignObjectRel doesn't have reverse_path_infos.
        foreign_object.remote_field.path_infos
        self.assertIn("path_infos", foreign_object.__dict__)
        self.assertIn("reverse_path_infos", foreign_object.__dict__)
        self.assertIn("path_infos", foreign_object.remote_field.__dict__)
        # Cached value is removed via __getstate__() on ForeignObjectRel
        # because no __deepcopy__() method exists, so __reduce_ex__() is used.
        remote_field_copy = copy.deepcopy(foreign_object.remote_field)
        self.assertNotIn("path_infos", remote_field_copy.__dict__)
        # Field.__deepcopy__() internally uses __copy__() on both the
        # ForeignObject and ForeignObjectRel, so all cached values are removed.
        foreign_object_copy = copy.deepcopy(foreign_object)
        self.assertNotIn("path_infos", foreign_object_copy.__dict__)
        self.assertNotIn("reverse_path_infos", foreign_object_copy.__dict__)
        self.assertNotIn("path_infos", foreign_object_copy.remote_field.__dict__)

    def test_pickling_foreignobjectrel(self):
        """
        Pickling a ForeignObjectRel removes the path_infos attribute.

        ForeignObjectRel implements __getstate__(), so copy and pickle modules
        both use that, but ForeignObject implements __reduce__() and __copy__()
        separately, so doesn't share the same behaviour.
        """
        foreign_object_rel = Membership._meta.get_field("person").remote_field
        # Trigger storage of cached_property into ForeignObjectRel's __dict__.
        foreign_object_rel.path_infos
        self.assertIn("path_infos", foreign_object_rel.__dict__)
        foreign_object_rel_restored = pickle.loads(pickle.dumps(foreign_object_rel))
        self.assertNotIn("path_infos", foreign_object_rel_restored.__dict__)

    def test_pickling_foreignobject(self):
        """
        Pickling a ForeignObject does not remove the cached PathInfo values.

        ForeignObject will always keep the path_infos and reverse_path_infos
        attributes within the same process, because of the way
        Field.__reduce__() is used for restoring values.
        """
        foreign_object = Membership._meta.get_field("person")
        # Trigger storage of cached_property into ForeignObjectRel's __dict__
        foreign_object.path_infos
        foreign_object.reverse_path_infos
        self.assertIn("path_infos", foreign_object.__dict__)
        self.assertIn("reverse_path_infos", foreign_object.__dict__)
        foreign_object_restored = pickle.loads(pickle.dumps(foreign_object))
        self.assertIn("path_infos", foreign_object_restored.__dict__)
        self.assertIn("reverse_path_infos", foreign_object_restored.__dict__)