public function containsResourceIdentifier($resourceIdentifier)
    {
        // This will be very slow however the base implementation can do nothing else.
        // Inheritors of this class should override this if they can do this faster!
        $items = $this->getItems(0, false);

        foreach ($items[0] as $item) {
            if ($item->_id = $resourceIdentifier) {
                return true;
            }
        }

        return false;
    }