How to know about user id in sessions in rails

I have a rails app. Now I want to fetch the users id to store it into the database. But system is showing error saying that id value is nill.

How can I store the user id in a integer or string variable?

 def create

 user = User.authenticate(params[:email], params[:password])
 @data=Information.all
 @num=Information.count()
 @tym=Time.now.getutc
 if user
  session[:user_id] = user.id
  @val=session[:user_id]               # is it possible to store user id in variable @val
  #redirect_to root_url, :notice => "Logged in!"

  #this part is for showing the content of the information table


  respond_to do |format|
  format.html
  format.xml { render xml: @data }
  end
else
  flash.now.alert = "Invalid email or password"
  render "new"
end
end