docs: def unassign(self):
          """Unassign the registered object.
  
          Note:
          Only registered PIDs can be redirected so we set it back to registered.
  
          :returns: `True` if the PID is successfully unassigned.
          """
          if self.object_uuid is None and self.object_type is None:
              return True
  
          try:
              with db.session.begin_nested():
                  if self.is_redirected():
                      db.session.delete(Redirect.query.get(self.object_uuid))
                      # Only registered PIDs can be redirected so we set it back
                      # to registered
                      self.status = PIDStatus.REGISTERED
                  self.object_type = None
                  self.object_uuid = None
                  db.session.add(self)
          except SQLAlchemyError:
              logger.exception("Failed to unassign object.".format(self),
                               extra=dict(pid=self))
              raise
          logger.info("Unassigned object from {0}.".format(self),
                      extra=dict(pid=self))
          return True