Python django.db.models.deletion.CASCADE Examples

def _test_m2m_create(self, M2MFieldClass):
        """
        Tests M2M fields on models during creation
        """
        class LocalBookWithM2M(Model):
            author = ForeignKey(Author, CASCADE)
            title = CharField(max_length=100, db_index=True)
            pub_date = DateTimeField()
            tags = M2MFieldClass("TagM2MTest", related_name="books")

            class Meta:
                app_label = 'schema'
                apps = new_apps
        self.local_models = [LocalBookWithM2M]
        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(TagM2MTest)
            editor.create_model(LocalBookWithM2M)
        # Ensure there is now an m2m table there
        columns = self.column_classes(LocalBookWithM2M._meta.get_field("tags").remote_field.through)
        self.assertEqual(columns['tagm2mtest_id'][0], "IntegerField") 

def test_alter_fk(self):
        """
        Tests altering of FKs
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        # Ensure the field is right to begin with
        columns = self.column_classes(Book)
        self.assertEqual(columns['author_id'][0], "IntegerField")
        self.assertForeignKeyExists(Book, 'author_id', 'schema_author')
        # Alter the FK
        old_field = Book._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE, editable=False)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        # Ensure the field is right afterwards
        columns = self.column_classes(Book)
        self.assertEqual(columns['author_id'][0], "IntegerField")
        self.assertForeignKeyExists(Book, 'author_id', 'schema_author') 

def test_alter_fk(self):
        """
        Tests altering of FKs
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        # Ensure the field is right to begin with
        columns = self.column_classes(Book)
        self.assertEqual(columns['author_id'][0], "IntegerField")
        self.assertForeignKeyExists(Book, 'author_id', 'schema_author')
        # Alter the FK
        old_field = Book._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE, editable=False)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        # Ensure the field is right afterwards
        columns = self.column_classes(Book)
        self.assertEqual(columns['author_id'][0], "IntegerField")
        self.assertForeignKeyExists(Book, 'author_id', 'schema_author') 

def test_fk_to_proxy(self):
        "Creating a FK to a proxy model creates database constraints."
        class AuthorProxy(Author):
            class Meta:
                app_label = 'schema'
                apps = new_apps
                proxy = True

        class AuthorRef(Model):
            author = ForeignKey(AuthorProxy, on_delete=CASCADE)

            class Meta:
                app_label = 'schema'
                apps = new_apps

        self.local_models = [AuthorProxy, AuthorRef]

        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(AuthorRef)
        self.assertForeignKeyExists(AuthorRef, 'author_id', 'schema_author') 

def test_fk(self):
        "Creating tables out of FK order, then repointing, works"
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Book)
            editor.create_model(Author)
            editor.create_model(Tag)
        # Initial tables are there
        list(Author.objects.all())
        list(Book.objects.all())
        # Make sure the FK constraint is present
        with self.assertRaises(IntegrityError):
            Book.objects.create(
                author_id=1,
                title="Much Ado About Foreign Keys",
                pub_date=datetime.datetime.now(),
            )
        # Repoint the FK constraint
        old_field = Book._meta.get_field("author")
        new_field = ForeignKey(Tag, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        self.assertForeignKeyExists(Book, 'author_id', 'schema_tag') 

def test_fk_to_proxy(self):
        "Creating a FK to a proxy model creates database constraints."
        class AuthorProxy(Author):
            class Meta:
                app_label = 'schema'
                apps = new_apps
                proxy = True

        class AuthorRef(Model):
            author = ForeignKey(AuthorProxy, on_delete=CASCADE)

            class Meta:
                app_label = 'schema'
                apps = new_apps

        self.local_models = [AuthorProxy, AuthorRef]

        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(AuthorRef)
        self.assertForeignKeyExists(AuthorRef, 'author_id', 'schema_author') 

def test_rename_referenced_field(self):
        class Author(Model):
            name = CharField(max_length=255, unique=True)

            class Meta:
                app_label = 'schema'

        class Book(Model):
            author = ForeignKey(Author, CASCADE, to_field='name')

            class Meta:
                app_label = 'schema'

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        new_field = CharField(max_length=255, unique=True)
        new_field.set_attributes_from_name('renamed')
        with connection.schema_editor(atomic=connection.features.supports_atomic_references_rename) as editor:
            editor.alter_field(Author, Author._meta.get_field('name'), new_field)
        # Ensure the foreign key reference was updated.
        self.assertForeignKeyExists(Book, 'author_id', 'schema_author', 'renamed') 

def __init__(self, to, to_field=None, rel_class=ManyToOneRel, **kwargs):
        try:
            to_name = to._meta.object_name.lower()
        except AttributeError:  # to._meta doesn't exist, so it must be RECURSIVE_RELATIONSHIP_CONSTANT
            assert isinstance(to, six.string_types), "%s(%r) is invalid. First parameter to ForeignKey must be either a model, a model name, or the string %r" % (self.__class__.__name__, to, RECURSIVE_RELATIONSHIP_CONSTANT)
        else:
            assert not to._meta.abstract, "%s cannot define a relation with abstract class %s" % (self.__class__.__name__, to._meta.object_name)
            # For backwards compatibility purposes, we need to *try* and set
            # the to_field during FK construction. It won't be guaranteed to
            # be correct until contribute_to_class is called. Refs #12190.
            to_field = to_field or (to._meta.pk and to._meta.pk.name)
        kwargs['verbose_name'] = kwargs.get('verbose_name', None)

        if 'db_index' not in kwargs:
            kwargs['db_index'] = True

        kwargs['rel'] = rel_class(to, to_field,
            related_name=kwargs.pop('related_name', None),
            limit_choices_to=kwargs.pop('limit_choices_to', None),
            parent_link=kwargs.pop('parent_link', False),
            on_delete=kwargs.pop('on_delete', CASCADE),
        )
        Field.__init__(self, **kwargs) 

def test_unique_together_with_fk_with_existing_index(self):
        """
        Tests removing and adding unique_together constraints that include
        a foreign key, where the foreign key is added after the model is
        created.
        """
        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithoutAuthor)
            new_field = ForeignKey(Author, CASCADE)
            new_field.set_attributes_from_name('author')
            editor.add_field(BookWithoutAuthor, new_field)
        # Ensure the fields aren't unique to begin with
        self.assertEqual(Book._meta.unique_together, ())
        # Add the unique_together constraint
        with connection.schema_editor() as editor:
            editor.alter_unique_together(Book, [], [['author', 'title']])
        # Alter it back
        with connection.schema_editor() as editor:
            editor.alter_unique_together(Book, [['author', 'title']], []) 

def __init__(self, to, from_fields, to_fields, swappable=True, **kwargs):
        self.from_fields = from_fields
        self.to_fields = to_fields
        self.swappable = swappable

        if 'rel' not in kwargs:
            kwargs['rel'] = ForeignObjectRel(
                self, to,
                related_name=kwargs.pop('related_name', None),
                related_query_name=kwargs.pop('related_query_name', None),
                limit_choices_to=kwargs.pop('limit_choices_to', None),
                parent_link=kwargs.pop('parent_link', False),
                on_delete=kwargs.pop('on_delete', CASCADE),
            )
        kwargs['verbose_name'] = kwargs.get('verbose_name', None)

        super(ForeignObject, self).__init__(**kwargs) 

def test_add_foreign_key_quoted_db_table(self):
        class Author(Model):
            class Meta:
                db_table = '"table_author_double_quoted"'
                app_label = 'schema'

        class Book(Model):
            author = ForeignKey(Author, CASCADE)

            class Meta:
                app_label = 'schema'

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        if connection.vendor == 'mysql':
            self.assertForeignKeyExists(Book, 'author_id', '"table_author_double_quoted"')
        else:
            self.assertForeignKeyExists(Book, 'author_id', 'table_author_double_quoted') 

def test_unique_together_with_fk_with_existing_index(self):
        """
        Tests removing and adding unique_together constraints that include
        a foreign key, where the foreign key is added after the model is
        created.
        """
        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithoutAuthor)
            new_field = ForeignKey(Author, CASCADE)
            new_field.set_attributes_from_name('author')
            editor.add_field(BookWithoutAuthor, new_field)
        # Ensure the fields aren't unique to begin with
        self.assertEqual(Book._meta.unique_together, ())
        # Add the unique_together constraint
        with connection.schema_editor() as editor:
            editor.alter_unique_together(Book, [], [['author', 'title']])
        # Alter it back
        with connection.schema_editor() as editor:
            editor.alter_unique_together(Book, [['author', 'title']], []) 

def test_rename_column_renames_deferred_sql_references(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
            old_title = Book._meta.get_field('title')
            new_title = CharField(max_length=100, db_index=True)
            new_title.set_attributes_from_name('renamed_title')
            editor.alter_field(Book, old_title, new_title)
            old_author = Book._meta.get_field('author')
            new_author = ForeignKey(Author, CASCADE)
            new_author.set_attributes_from_name('renamed_author')
            editor.alter_field(Book, old_author, new_author)
            self.assertGreater(len(editor.deferred_sql), 0)
            for statement in editor.deferred_sql:
                self.assertIs(statement.references_column('book', 'title'), False)
                self.assertIs(statement.references_column('book', 'author_id'), False) 

def test_fk(self):
        "Creating tables out of FK order, then repointing, works"
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Book)
            editor.create_model(Author)
            editor.create_model(Tag)
        # Initial tables are there
        list(Author.objects.all())
        list(Book.objects.all())
        # Make sure the FK constraint is present
        with self.assertRaises(IntegrityError):
            Book.objects.create(
                author_id=1,
                title="Much Ado About Foreign Keys",
                pub_date=datetime.datetime.now(),
            )
        # Repoint the FK constraint
        old_field = Book._meta.get_field("author")
        new_field = ForeignKey(Tag, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        self.assertForeignKeyExists(Book, 'author_id', 'schema_tag') 

def test_char_field_with_db_index_to_fk(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(AuthorCharFieldWithIndex)
        # Change CharField to FK
        old_field = AuthorCharFieldWithIndex._meta.get_field('char_field')
        new_field = ForeignKey(Author, CASCADE, blank=True)
        new_field.set_attributes_from_name('char_field')
        with connection.schema_editor() as editor:
            editor.alter_field(AuthorCharFieldWithIndex, old_field, new_field, strict=True)
        self.assertForeignKeyExists(AuthorCharFieldWithIndex, 'char_field_id', 'schema_author') 

def test_no_index_for_foreignkey(self):
        """
        MySQL on InnoDB already creates indexes automatically for foreign keys.
        (#14180). An index should be created if db_constraint=False (#26171).
        """
        storage = connection.introspection.get_storage_engine(
            connection.cursor(), ArticleTranslation._meta.db_table
        )
        if storage != "InnoDB":
            self.skip("This test only applies to the InnoDB storage engine")
        index_sql = [str(statement) for statement in connection.schema_editor()._model_indexes_sql(ArticleTranslation)]
        self.assertEqual(index_sql, [
            'CREATE INDEX `indexes_articletranslation_article_no_constraint_id_d6c0806b` '
            'ON `indexes_articletranslation` (`article_no_constraint_id`)'
        ])

        # The index also shouldn't be created if the ForeignKey is added after
        # the model was created.
        field_created = False
        try:
            with connection.schema_editor() as editor:
                new_field = ForeignKey(Article, CASCADE)
                new_field.set_attributes_from_name('new_foreign_key')
                editor.add_field(ArticleTranslation, new_field)
                field_created = True
                self.assertEqual([str(statement) for statement in editor.deferred_sql], [
                    'ALTER TABLE `indexes_articletranslation` '
                    'ADD CONSTRAINT `indexes_articletrans_new_foreign_key_id_d27a9146_fk_indexes_a` '
                    'FOREIGN KEY (`new_foreign_key_id`) REFERENCES `indexes_article` (`id`)'
                ])
        finally:
            if field_created:
                with connection.schema_editor() as editor:
                    editor.remove_field(ArticleTranslation, new_field) 

def test_text_field_with_db_index_to_fk(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(AuthorTextFieldWithIndex)
        # Change TextField to FK
        old_field = AuthorTextFieldWithIndex._meta.get_field('text_field')
        new_field = ForeignKey(Author, CASCADE, blank=True)
        new_field.set_attributes_from_name('text_field')
        with connection.schema_editor() as editor:
            editor.alter_field(AuthorTextFieldWithIndex, old_field, new_field, strict=True)
        self.assertForeignKeyExists(AuthorTextFieldWithIndex, 'text_field_id', 'schema_author') 

def test_no_db_constraint_added_during_primary_key_change(self):
        """
        When a primary key that's pointed to by a ForeignKey with
        db_constraint=False is altered, a foreign key constraint isn't added.
        """
        class Author(Model):
            class Meta:
                app_label = 'schema'

        class BookWeak(Model):
            author = ForeignKey(Author, CASCADE, db_constraint=False)

            class Meta:
                app_label = 'schema'

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWeak)
        self.assertForeignKeyNotExists(BookWeak, 'author_id', 'schema_author')
        old_field = Author._meta.get_field('id')
        new_field = BigAutoField(primary_key=True)
        new_field.model = Author
        new_field.set_attributes_from_name('id')
        # @isolate_apps() and inner models are needed to have the model
        # relations populated, otherwise this doesn't act as a regression test.
        self.assertEqual(len(new_field.model._meta.related_objects), 1)
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertForeignKeyNotExists(BookWeak, 'author_id', 'schema_author') 

def test_alter_fk_checks_deferred_constraints(self):
        """
        #25492 - Altering a foreign key's structure and data in the same
        transaction.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Node)
        old_field = Node._meta.get_field('parent')
        new_field = ForeignKey(Node, CASCADE)
        new_field.set_attributes_from_name('parent')
        parent = Node.objects.create()
        with connection.schema_editor() as editor:
            # Update the parent FK to create a deferred constraint check.
            Node.objects.update(parent=parent)
            editor.alter_field(Node, old_field, new_field, strict=True) 

def test_alter_to_fk(self):
        """
        #24447 - Tests adding a FK constraint for an existing column
        """
        class LocalBook(Model):
            author = IntegerField()
            title = CharField(max_length=100, db_index=True)
            pub_date = DateTimeField()

            class Meta:
                app_label = 'schema'
                apps = new_apps

        self.local_models = [LocalBook]

        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(LocalBook)
        # Ensure no FK constraint exists
        constraints = self.get_constraints(LocalBook._meta.db_table)
        for details in constraints.values():
            if details['foreign_key']:
                self.fail('Found an unexpected FK constraint to %s' % details['columns'])
        old_field = LocalBook._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(LocalBook, old_field, new_field, strict=True)
        self.assertForeignKeyExists(LocalBook, 'author_id', 'schema_author') 

def _test_m2m_through_alter(self, M2MFieldClass):
        """
        Tests altering M2Ms with explicit through models (should no-op)
        """
        class LocalAuthorTag(Model):
            author = ForeignKey("schema.LocalAuthorWithM2MThrough", CASCADE)
            tag = ForeignKey("schema.TagM2MTest", CASCADE)

            class Meta:
                app_label = 'schema'
                apps = new_apps

        class LocalAuthorWithM2MThrough(Model):
            name = CharField(max_length=255)
            tags = M2MFieldClass("schema.TagM2MTest", related_name="authors", through=LocalAuthorTag)

            class Meta:
                app_label = 'schema'
                apps = new_apps

        self.local_models = [LocalAuthorTag, LocalAuthorWithM2MThrough]

        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(LocalAuthorTag)
            editor.create_model(LocalAuthorWithM2MThrough)
            editor.create_model(TagM2MTest)
        # Ensure the m2m table is there
        self.assertEqual(len(self.column_classes(LocalAuthorTag)), 3)
        # "Alter" the field's blankness. This should not actually do anything.
        old_field = LocalAuthorWithM2MThrough._meta.get_field("tags")
        new_field = M2MFieldClass("schema.TagM2MTest", related_name="authors", through=LocalAuthorTag)
        new_field.contribute_to_class(LocalAuthorWithM2MThrough, "tags")
        with connection.schema_editor() as editor:
            editor.alter_field(LocalAuthorWithM2MThrough, old_field, new_field, strict=True)
        # Ensure the m2m table is still there
        self.assertEqual(len(self.column_classes(LocalAuthorTag)), 3) 

def test_alter_o2o_to_fk(self):
        """
        #24163 - Tests altering of OneToOneField to ForeignKey
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithO2O)
        # Ensure the field is right to begin with
        columns = self.column_classes(BookWithO2O)
        self.assertEqual(columns['author_id'][0], "IntegerField")
        # Ensure the field is unique
        author = Author.objects.create(name="Joe")
        BookWithO2O.objects.create(author=author, title="Django 1", pub_date=datetime.datetime.now())
        with self.assertRaises(IntegrityError):
            BookWithO2O.objects.create(author=author, title="Django 2", pub_date=datetime.datetime.now())
        BookWithO2O.objects.all().delete()
        self.assertForeignKeyExists(BookWithO2O, 'author_id', 'schema_author')
        # Alter the OneToOneField to ForeignKey
        old_field = BookWithO2O._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithO2O, old_field, new_field, strict=True)
        # Ensure the field is right afterwards
        columns = self.column_classes(Book)
        self.assertEqual(columns['author_id'][0], "IntegerField")
        # Ensure the field is not unique anymore
        Book.objects.create(author=author, title="Django 1", pub_date=datetime.datetime.now())
        Book.objects.create(author=author, title="Django 2", pub_date=datetime.datetime.now())
        self.assertForeignKeyExists(Book, 'author_id', 'schema_author') 

def test_alter_field_fk_to_o2o(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        expected_fks = 1 if connection.features.supports_foreign_keys else 0

        # Check the index is right to begin with.
        counts = self.get_constraints_count(
            Book._meta.db_table,
            Book._meta.get_field('author').column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        self.assertEqual(counts, {'fks': expected_fks, 'uniques': 0, 'indexes': 1})

        old_field = Book._meta.get_field('author')
        new_field = OneToOneField(Author, CASCADE)
        new_field.set_attributes_from_name('author')
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)

        counts = self.get_constraints_count(
            Book._meta.db_table,
            Book._meta.get_field('author').column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        # The index on ForeignKey is replaced with a unique constraint for OneToOneField.
        self.assertEqual(counts, {'fks': expected_fks, 'uniques': 1, 'indexes': 0}) 

def test_alter_field_fk_keeps_index(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        expected_fks = 1 if connection.features.supports_foreign_keys else 0

        # Check the index is right to begin with.
        counts = self.get_constraints_count(
            Book._meta.db_table,
            Book._meta.get_field('author').column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        self.assertEqual(counts, {'fks': expected_fks, 'uniques': 0, 'indexes': 1})

        old_field = Book._meta.get_field('author')
        # on_delete changed from CASCADE.
        new_field = ForeignKey(Author, PROTECT)
        new_field.set_attributes_from_name('author')
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)

        counts = self.get_constraints_count(
            Book._meta.db_table,
            Book._meta.get_field('author').column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        # The index remains.
        self.assertEqual(counts, {'fks': expected_fks, 'uniques': 0, 'indexes': 1}) 

def test_alter_field_o2o_to_fk(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithO2O)
        expected_fks = 1 if connection.features.supports_foreign_keys else 0

        # Check the unique constraint is right to begin with.
        counts = self.get_constraints_count(
            BookWithO2O._meta.db_table,
            BookWithO2O._meta.get_field('author').column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        self.assertEqual(counts, {'fks': expected_fks, 'uniques': 1, 'indexes': 0})

        old_field = BookWithO2O._meta.get_field('author')
        new_field = ForeignKey(Author, CASCADE)
        new_field.set_attributes_from_name('author')
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithO2O, old_field, new_field, strict=True)

        counts = self.get_constraints_count(
            BookWithO2O._meta.db_table,
            BookWithO2O._meta.get_field('author').column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        # The unique constraint on OneToOneField is replaced with an index for ForeignKey.
        self.assertEqual(counts, {'fks': expected_fks, 'uniques': 0, 'indexes': 1}) 

def test_alter_field_o2o_keeps_unique(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithO2O)
        expected_fks = 1 if connection.features.supports_foreign_keys else 0

        # Check the unique constraint is right to begin with.
        counts = self.get_constraints_count(
            BookWithO2O._meta.db_table,
            BookWithO2O._meta.get_field('author').column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        self.assertEqual(counts, {'fks': expected_fks, 'uniques': 1, 'indexes': 0})

        old_field = BookWithO2O._meta.get_field('author')
        # on_delete changed from CASCADE.
        new_field = OneToOneField(Author, PROTECT)
        new_field.set_attributes_from_name('author')
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithO2O, old_field, new_field, strict=True)

        counts = self.get_constraints_count(
            BookWithO2O._meta.db_table,
            BookWithO2O._meta.get_field('author').column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        # The unique constraint remains.
        self.assertEqual(counts, {'fks': expected_fks, 'uniques': 1, 'indexes': 0}) 

def _test_m2m_create_through(self, M2MFieldClass):
        """
        Tests M2M fields on models during creation with through models
        """
        class LocalTagThrough(Model):
            book = ForeignKey("schema.LocalBookWithM2MThrough", CASCADE)
            tag = ForeignKey("schema.TagM2MTest", CASCADE)

            class Meta:
                app_label = 'schema'
                apps = new_apps

        class LocalBookWithM2MThrough(Model):
            tags = M2MFieldClass("TagM2MTest", related_name="books", through=LocalTagThrough)

            class Meta:
                app_label = 'schema'
                apps = new_apps

        self.local_models = [LocalTagThrough, LocalBookWithM2MThrough]

        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(LocalTagThrough)
            editor.create_model(TagM2MTest)
            editor.create_model(LocalBookWithM2MThrough)
        # Ensure there is now an m2m table there
        columns = self.column_classes(LocalTagThrough)
        self.assertEqual(columns['book_id'][0], "IntegerField")
        self.assertEqual(columns['tag_id'][0], "IntegerField") 

def _test_m2m_through_alter(self, M2MFieldClass):
        """
        Tests altering M2Ms with explicit through models (should no-op)
        """
        class LocalAuthorTag(Model):
            author = ForeignKey("schema.LocalAuthorWithM2MThrough", CASCADE)
            tag = ForeignKey("schema.TagM2MTest", CASCADE)

            class Meta:
                app_label = 'schema'
                apps = new_apps

        class LocalAuthorWithM2MThrough(Model):
            name = CharField(max_length=255)
            tags = M2MFieldClass("schema.TagM2MTest", related_name="authors", through=LocalAuthorTag)

            class Meta:
                app_label = 'schema'
                apps = new_apps

        self.local_models = [LocalAuthorTag, LocalAuthorWithM2MThrough]

        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(LocalAuthorTag)
            editor.create_model(LocalAuthorWithM2MThrough)
            editor.create_model(TagM2MTest)
        # Ensure the m2m table is there
        self.assertEqual(len(self.column_classes(LocalAuthorTag)), 3)
        # "Alter" the field's blankness. This should not actually do anything.
        old_field = LocalAuthorWithM2MThrough._meta.get_field("tags")
        new_field = M2MFieldClass("schema.TagM2MTest", related_name="authors", through=LocalAuthorTag)
        new_field.contribute_to_class(LocalAuthorWithM2MThrough, "tags")
        with connection.schema_editor() as editor:
            editor.alter_field(LocalAuthorWithM2MThrough, old_field, new_field, strict=True)
        # Ensure the m2m table is still there
        self.assertEqual(len(self.column_classes(LocalAuthorTag)), 3) 

def test_unique_no_unnecessary_fk_drops(self):
        """
        If AlterField isn't selective about dropping foreign key constraints
        when modifying a field with a unique constraint, the AlterField
        incorrectly drops and recreates the Book.author foreign key even though
        it doesn't restrict the field being changed (#29193).
        """
        class Author(Model):
            name = CharField(max_length=254, unique=True)

            class Meta:
                app_label = 'schema'

        class Book(Model):
            author = ForeignKey(Author, CASCADE)

            class Meta:
                app_label = 'schema'

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        new_field = CharField(max_length=255, unique=True)
        new_field.model = Author
        new_field.set_attributes_from_name('name')
        with self.assertLogs('django.db.backends.schema', 'DEBUG') as cm:
            with connection.schema_editor() as editor:
                editor.alter_field(Author, Author._meta.get_field('name'), new_field)
        # One SQL statement is executed to alter the field.
        self.assertEqual(len(cm.records), 1) 

def test_add_foreign_key_long_names(self):
        """
        Regression test for #23009.
        Only affects databases that supports foreign keys.
        """
        # Create the initial tables
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithEvenLongerName)
            editor.create_model(BookWithLongName)
        # Add a second FK, this would fail due to long ref name before the fix
        new_field = ForeignKey(AuthorWithEvenLongerName, CASCADE, related_name="something")
        new_field.set_attributes_from_name("author_other_really_long_named_i_mean_so_long_fk")
        with connection.schema_editor() as editor:
            editor.add_field(BookWithLongName, new_field)