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