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

    public function getDocumentIdentifier($document)
    {
        return isset($this->documentIdentifiers[spl_object_hash($document)]) ? $this->documentIdentifiers[spl_object_hash($document)] : null;
    }

 /**
  * Releases any lock that exists on this document.
  *
  * @param object $document
  */
 public function unlock($document)
 {
     $criteria = array('_id' => $this->uow->getDocumentIdentifier($document));
     $lockMapping = $this->class->fieldMappings[$this->class->lockField];
     $this->collection->update($criteria, array($this->cmd.'unset' => array($lockMapping['name'] => true)));
     $this->class->reflFields[$this->class->lockField]->setValue($document, null);
 }