Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'tones') and self.tones is not None:
            _dict['tones'] = [x._to_dict() for x in self.tones]
        if hasattr(self, 'category_id') and self.category_id is not None:
            _dict['category_id'] = self.category_id
        if hasattr(self, 'category_name') and self.category_name is not None:
            _dict['category_name'] = self.category_name
        return _dict