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