code: func (b *buildEntity) getID() string {
  	s := []string{b.Master, b.Buildername, strconv.Itoa(b.Number)}
  	id, err := json.Marshal(s)
  	if err != nil {
  		panic(err) // This can't fail.
  	}
  	return string(id)
  }
docs: // getID is a helper function that returns b's datastore key.