docs: def get_resource(self):
          """Return the associated resource."""
          references = {"resource_id": None, "parent_id": None,
                        "grandparent_id": None}
          for model_cls, regexp in self._regexp.iteritems():
              match = regexp.search(self.resource_ref)
              if match is not None:
                  references.update(match.groupdict())
                  return model_cls.get(**references)
  
          raise exception.NotFound("No model available for %(resource_ref)r",
                                   resource_ref=self.resource_ref)