code: func (ms *modelSpec) modelKey(id string) (string, error) {
  	if id == "" {
  		return "", fmt.Errorf("zoom: Error in modelKey: id was empty")
  	}
  	return ms.name + ":" + id, nil
  }
docs: // modelKey returns the key that identifies a hash in the database
  // which contains all the fields of the model corresponding to the given
  // id. It returns an error iff id is empty.