code: def update(self, instance):
          
          assert isinstance(instance, UnitOfWork)
          if instance.db_id:
              query = {'_id': ObjectId(instance.db_id)}
          else:
              query = {unit_of_work.PROCESS_NAME: instance.process_name,
                       unit_of_work.TIMEPERIOD: instance.timeperiod,
                       unit_of_work.START_ID: instance.start_id,
                       unit_of_work.END_ID: instance.end_id}
          self.ds.update(COLLECTION_UNIT_OF_WORK, query, instance)
          return instance.db_id
docs: method finds unit_of_work record and change its status