public function getCacheKey()
    {
        $cacheKey = implode('_', array(
            $this->ID,
            strtotime($this->LastEdited),
            strtotime($this->Parent()->LastEdited),
            Versioned::get_reading_mode()
        ));
        $this->extend('updateCacheKey', $cacheKey);
        return $cacheKey;
    }