Propagate to nested fields.

        :returns: data dictionary or ``None`` if no fields are present.

def serialize(self, value, entity, request):
        """ Propagate to nested fields.

        :returns: data dictionary or ``None`` if no fields are present.
        """

        self._validate_existence(value)
        self._run_validators(value)

        if not value:
            return value

        return self.factory.serialize(value, request)