docs: def index(self, child_pid):
          """Index of the child in the relation."""
          if not isinstance(child_pid, PersistentIdentifier):
              child_pid = resolve_pid(child_pid)
          relation = PIDRelation.query.filter_by(
              parent=self._resolved_pid,
              child=child_pid,
              relation_type=self.relation_type.id).one()
          return relation.index