code: func (c *Collection) ModelKey(id string) string {
  	if id == "" {
  		return ""
  	}
  	// c.spec.modelKey(id) will only return an error if id was an empty string.
  	// Since we already ruled that out with the check above, we can safely ignore
  	// the error return value here.
  	key, _ := c.spec.modelKey(id)
  	return key
  }
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. If id is an empty string, it will return an empty string.