Added a unit test to ensure models cannot be ordered by a M2M field.

        ]

        self.assertEqual(errors, expected)

    def test_non_valid(self):

        class RelationModel(models.Model):

            pass

        class Model(models.Model):

            relation = models.ManyToManyField(RelationModel)

            class Meta:

                ordering = ['relation']

        errors = Model.check()

        expected = [

            Error(

                "'ordering' refers to the non-existent field 'relation'.",

                hint=None,

                obj=Model,

                id='models.E015',

            ),

        ]

        self.assertEqual(errors, expected)

    def test_ordering_pointing_to_missing_field(self):

        class Model(models.Model):

            class Meta: