public function render()
    {
        $value = $this->arguments['value'];

        if ($value === null) {
            $value = $this->renderChildren();
        }
        if ($value === null) {
            return null;
        }
        if (!is_object($value)) {
            throw new ViewHelper\Exception('f:format.identifier expects an object, ' . gettype($value) . ' given.', 1337700024);
        }
        return $this->persistenceManager->getIdentifierByObject($value);
    }