docs: 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