public function getSortableKey($key = null)
    {
        if ( ! is_null($key))
        {
            return $this->determineSortableKey($key);
        }

        return $this->getDefaultSortableKey();
    }
public function getSortableKey($key = null)
    {
        if ( ! is_null($key))
        {
            return $this->determineSortableKey($key);
        }

        return $this->getDefaultSortableKey();
    }
public function getSortableKey ( $ key = null ) { if ( ! is_null ( $ key ) ) { return $ this -> determineSortableKey ( $ key ) ; } return $ this -> getDefaultSortableKey ( ) ; }
Returns default key if isset or validates key

@param string|null $key
@return string
Returns default key if isset or validates key