Return a json dictionary representing this model.

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