def METHOD_NAME(self):
    nested = Nested.objects.create(name=faker.name())
    form = self.form.from_json({'nested': {
        'class': 'Bad',
        'id': str(nested.id)
    }})
    form.validate()
    self.assertIn('nested', form.errors)
    self.assertEqual(len(form.errors['nested']), 1)