Return a json dictionary representing this model.

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