docs: 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