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