Ruby on Rails 4.0 session[cart_id] = nil - ruby-on-rails-4

Question

In my process of learning RoR 4.0, I am following a tutorial to build a Rails project where a Cart is being implemented. My problem is that I can't place more than one item on my cart, because when I go back to the main page to select another item, my cart changes its id . I have the cart_id stored on a session variable, on a function on my application controller:
class ApplicationController < ActionController::Base
protect_from_forgery
private
def current_cart
if session[cart_id].nil? (*)
puts “ ********************* “ (*)
puts session[cart_id] (*)
end
Cart.find(session[:cart_id])
rescue ActiveRecord::RecordNotFound
cart = Cart.create
session[:cart_id] = cart.id
cart
end
end
I call this function from this controller as follows:
class LineItemsController < ApplicationController
def create
#cart = current_cart
product = Product.find(params[:product_id])
#line_item = #cart.line_items.build(:product => product)
respond_to do |format|
if #line_item.save
format.html { redirect_to(#line_item.cart,
:notice => 'Line item was successfully created.') }
format.xml { render :xml => #line_item,
:status => :created, :location => #line_item }
else
format.html { render :action => "new" }
format.xml { render :xml => #line_item.errors,
:status => :unprocessable_entity }
end
end
end
I made several testes, added the three lines of code marked as (*) to my current_cart and I notice that every time that #cart = currect_cart is executed on my lineItems Controller, session[cart_id] is nil.
How could this happen? can comeone explain this to me?
Thank you in advance for your help !