Return a json dictionary representing this model.

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