Iteration F2.

app/controllers/line_items_controller.rb

@@ -34,6 +34,7 @@ class LineItemsController < ApplicationController
     respond_to do |format|
       if @line_item.save
         format.html { redirect_to @line_item.cart }
+        format.js
         format.json { render :show, status: :created, location: @line_item }
       else
         format.html { render :new }

app/views/line_items/create.js.erb

@@ -0,0 +1 @@
+$('#cart').html("<%=j render(@cart)  %>")

app/views/store/index.html.erb

@@ -11,7 +11,7 @@
         <%= sanitize(product.description) %>
         <div class="price_line">
           <span class="price"><%= number_to_currency(product.price) %></span>
-          <%= button_to 'Add To Cart', line_items_path(product_id: product) %>
+          <%= button_to 'Add To Cart', line_items_path(product_id: product), remote: true %>
         </div>
       </div>
     <% end %>