docs: def _to_dict(self):
          """Return a json dictionary representing this model."""
          _dict = {}
          if hasattr(self, 'key') and self.key is not None:
              _dict['key'] = self.key._to_dict()
          if hasattr(self, 'value') and self.value is not None:
              _dict['value'] = self.value._to_dict()
          return _dict