code: def _add_complete_key_entity_pb(self):
          
          # We use ``upsert`` for entities with completed keys, rather than
          # ``insert`` or ``update``, in order not to create race conditions
          # based on prior existence / removal of the entity.
          new_mutation = _datastore_pb2.Mutation()
          self._mutations.append(new_mutation)
          return new_mutation.upsert
docs: Adds a new mutation for an entity with a completed key.
  
          :rtype: :class:`.entity_pb2.Entity`
          :returns: The newly created entity protobuf that will be
                    updated and sent with a commit.