public function getNormalizedIdentifier($document)
    {
        if (is_scalar($document)) {
            throw new \InvalidArgumentException('Invalid argument, object or null required');
        }

        // the document is not managed
        if (!$document || !$this->getDocumentManager()->contains($document)) {
            return;
        }

        $values = $this->getIdentifierValues($document);

        return $values[0];
    }