code: def _get_extension_by_related_name(self, related_name):
          
          if related_name is None:
              return self._extensions[0]
  
          for meta in self._extensions:
              if meta.rel_name == related_name:
                  return meta
  
          raise ValueError("No translated model of '{0}' has a reverse name of '{1}'".format(
              self.root.shared_model.__name__, related_name
          ))
docs: Find which model is connected to a given related name.
          If the related name is ``None``, the :attr:`root_model` will be returned.