Return a json dictionary representing this model.

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