Return a json dictionary representing this model.

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