Doctrine\ODM\MongoDB\UnitOfWork::getIdForIdentityMap PHP Method

    private function getIdForIdentityMap($document)
    {
        $class = $this->dm->getClassMetadata(get_class($document));
        if (!$class->identifier) {
            $id = spl_object_hash($document);
        } else {
            $id = $this->documentIdentifiers[spl_object_hash($document)];
            $id = serialize($class->getDatabaseIdentifierValue($id));
        }
        return $id;
    }