public function add($key, & $var, $lifetime= 0, $options= array()) {
        $return= false;
        if ($cache = $this->getCacheProvider($this->getOption(xPDO::OPT_CACHE_KEY, $options))) {
            $value= null;
            if (is_object($var) && $var instanceof \xPDO\Om\xPDOObject) {
                $value= $var->toArray('', true);
            } else {
                $value= $var;
            }
            $return= $cache->add($key, $value, $lifetime, $options);
        }
        return $return;
    }