cakephp：facebook登录的会话问题

Question

I have implemented FB login in my app. For noraml login session is working fine. But when login with Fb and try to write data and redirect back to home page session is showing null.

// fb login successfull
if(!empty( $result )){
   $this->Session->write('user', $result['User']);
   //debug($this->Session->read('user'));
   $this->redirect("/home");
   }else{
// error
}

At home controller

function index(){
debug($this->Session->read('user'));
}

it is showing null. But when i put session_start() at top then session data is available. What's wrong with cakephp session.