public function order($spec)
    {
        if (!is_array($spec)) {
            $spec = array($spec);
        }

        // force 'ASC' or 'DESC' on each order spec, default is ASC.
        foreach ($spec as $val) {
            if ($val instanceof Zend_Db_Expr) {
                $expr = $val->__toString();
                if (empty($expr)) {
                    continue;
                }
                $this->_parts[self::ORDER][] = $val;
            } else {
                if (empty($val)) {
                    continue;
                }
                $direction = self::SQL_ASC;
                if (preg_match('/(.*\W)(' . self::SQL_ASC . '|' . self::SQL_DESC . ')\b/si', $val, $matches)) {
                    $val = trim($matches[1]);
                    $direction = $matches[2];
                }
                if (preg_match('/\(.*\)/', $val)) {
                    $val = new Zend_Db_Expr($val);
                }
                $this->_parts[self::ORDER][] = array($val, $direction);
            }
        }

        return $this;
    }