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