Return a json dictionary representing this model.

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