def METHOD_NAME(self, instance):
    super().METHOD_NAME(instance)
    # run extra uniqueness validation - since content_type and object_id
    # fields are not exposed through API (we're using nested resources),
    # DRF does not handle this validation automatically
    try:
        instance.validate_unique()
    except DjangoValidationError as e:
        raise self._django_validation_error_to_drf_validation_error(e)