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