code: func (b *buildEntity) setID(id string) error {
  	var s []string
  	err := json.Unmarshal([]byte(id), &s)
  	if err != nil {
  		return err
  	}
  	if len(s) != 3 {
  		return fmt.Errorf("%q does not have 3 items", id)
  	}
  	b.Master = s[0]
  	b.Buildername = s[1]
  	b.Number, err = strconv.Atoi(s[2])
  	return err // or nil.
  }
docs: // setID is the inverse of getID().