def get_rel_attr(self, attr_name, model):
        """For a related attribute specification, returns (related model,
        attribute).

        Returns (None, None) if model is not found, or (model, None) if
        attribute is not found.
        """
        rel_attr_name, attr_name = attr_name.split(".", 1)
        rel_attr = getattr(self.model, rel_attr_name, None)
        rel_model = None
        attr = None

        if rel_attr is not None:
            rel_model = rel_attr.property.mapper.class_
            attr = getattr(rel_model, attr_name, None)

        return rel_model, attr