code: def insert(self, instance):
          
          assert isinstance(instance, UnitOfWork)
          collection = self.ds.connection(COLLECTION_UNIT_OF_WORK)
          try:
              return collection.insert_one(instance.document).inserted_id
          except MongoDuplicateKeyError as e:
              exc = DuplicateKeyError(instance.process_name,
                                      instance.start_timeperiod,
                                      instance.start_id,
                                      instance.end_id,
                                      e)
              raise exc
docs: inserts a unit of work into MongoDB.
          :raises DuplicateKeyError: if such record already exist