Errors model abstract testing

[Artem]

I have models:

class Country(models.Model):
    name = models.CharField(_("Country name"), max_length=50)
    code = models.SlugField(_("Country iso-2 code"), max_length=2, unique=True)

class CountryModelMixin(models.Model):
    countries_admin = models.ManyToManyField(
        Country,
        verbose_name="Страны",
        related_name="%(app_label)s_%(class)s_related",
        related_query_name="%(app_label)s_%(class)ss",
    )
    countries = ArrayField(
        models.SlugField(verbose_name=_("Country iso-2 code"), max_length=2),
        verbose_name=_("Countries"),
        blank=True, null=True,
    )

    class Meta(object):
        abstract = True

    @staticmethod
    def update_hidden(sender, instance, **kwargs):
        if issubclass(instance.__class__, CountryModelMixin):
               instance.countries = countries.countries_admin.values_list('code', flat=True)

m2m_changed.connect(CountryModelMixin.update_hidden)

class Orgaization(CountryModelMixin):
    name = models.CharField(_("Name"), max_length=127)
    slug = models.SlugField(_("Slug"), max_length=127, unique=True)
    active = models.BooleanField(_("Active"), default=True)

    address = models.CharField(_("Address"), max_length=256, null=True, blank=True)

And I need testing CountryModelMixin models:
I create test:

class ModelMixinTestCase(TestCase):
    mixin= CountryModelMixin

    @classmethod
    def setUpClass(cls):
        if not cls.mixin:
            return
        if isinstance(cls.mixin, list):
            name = ""
            for mxn in cls.mixin:
                name += mxn.__name__
            name = hashlib.md5(name.encode('UTF-8')).hexdigest()
            cls.model = ModelBase('__TestModel__' + name, tuple(cls.mixin),
                                  {'__module__': cls.mixin[0].__module__})
        else:
            cls.model = ModelBase('__TestModel__' +
                cls.mixin.__name__, (cls.mixin,),
                {'__module__': cls.mixin.__module__}
            )

        # Create the schema for  our test model
        with connection.schema_editor() as schema_editor:
            schema_editor.create_model(cls.model)
        super(ModelMixinTestCase, cls).setUpClass()

    @classmethod
    def tearDownClass(cls):
        if not cls.mixin:
            return
        # Delete the schema for the test model
        with connection.schema_editor() as schema_editor:
            schema_editor.delete_model(cls.model)
        super(ModelMixinTestCase, cls).tearDownClass()

       def setUp(self):
               self.country = Country.objects.create(name="Test", code="EN")

       def test_go(self):
            myModelElement = self.model.objects.create()
            myModelElement.countries_admin.set([self.country.pk, ])

return self.instance._prefetched_objects_cache[self.prefetch_cache_name]
AttributeError: 'TestModel35ecfa746aba30e3344f3534dd7552ac' object has no attribute '_prefetched_objects_cache'

Test return errors: Cannot resolve keyword "" into field.
And delete myModelElement.delete() - don't work

Test abstract models without ManyToManyField work goods.