Session tracking in rails

Currently i am trying to implement session tracking in rails,
till now i was using

"Rack::Session::__id_ "

so i was getting different session id's when i restart my rails server, but as i want to implement the tracking on user's session so i want a unique id whenever a new user starts his session.
i have tried a lot things but not getting anything as

session[:session_id]  
session['session_id']  
session[:id]  
session['id']  
session.id  
session.session_id  

those are giving NIL values,