docs: def create(cls, parent, child, relation_type, index=None):
          """Create a PID relation for given parent and child."""
          try:
              with db.session.begin_nested():
                  obj = cls(parent_id=parent.id,
                            child_id=child.id,
                            relation_type=relation_type,
                            index=index)
                  db.session.add(obj)
          except IntegrityError:
              raise Exception("PID Relation already exists.")
              # msg = "PIDRelation already exists: " \
              #       "{0} -> {1} ({2})".format(
              #         parent_pid, child_pid, relation_type)
              # logger.exception(msg)
              # raise Exception(msg)
          return obj