code: def _add_partial_key_entity_pb(self):
          
          new_mutation = _datastore_pb2.Mutation()
          self._mutations.append(new_mutation)
          return new_mutation.insert
docs: Adds a new mutation for an entity with a partial key.
  
          :rtype: :class:`.entity_pb2.Entity`
          :returns: The newly created entity protobuf that will be
                    updated and sent with a commit.