How Can I Get _rev field in CouchDB Doctrine. Check if the document has changed

Question

I use Doctrine for my CocuhDb

I retrieve My Object by

$dm = $this->container->get('doctrine_couchdb.odm.default_document_manager');
$obj = $dm->getRepository("myEntity")->findOneBy(array('title' => 'foo));

I tried with (pseudo-code)

for {
.....
  $dm->persist($obj);
  $uow = $dm->getUnitOfWork();
  $changeset = $uow->getDocumentRevision($obj);
}

$dm->flush()

but my $obj has the old _rev.

I want to check if my document change _rev.

How Can I retrieve that information ?