如何在FOSUSER登录symfony2.6后保留以前设置的会话

Question

Hello I am working on shopping cart using Symfony2.6 with FOSUSer Bundle. And I have set session Id with Product id in sessionTable. Like this :-

    $session = $this->get('session');
    $session->start();
    $sessionId = $session->getId(); 


    $sessionTable = new SessionTable();
    $sessionTable->setSessionId($sessionId);
    $sessionTable->setProductId($productId);
    $em->persist($sessionTable);
    $em->flush();

Everything is working But When User Login then SessionId destroy. How I can get old Session Id after login or register ????