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