app/controllers/books_controller.rb

@@ -25,6 +25,8 @@ class BooksController < ApplicationController
 
   def add_to_cart
     @book = Book.find(params[:id])
+    return unless @book.quantity.positive?
+
     current_user.books << @book
     @book.decrement!(:quantity)
     redirect_to '/books', notice: 'Book added to your cart'

app/helpers/books_helper.rb

@@ -1,2 +1,8 @@
 module BooksHelper
+  def cart_summary
+    number_to_currency(@books.map{ |b| b.price }.reduce(:+))
+  end
+  def can_book_be_added?(book)
+    logged_in? && book.quantity.positive? && !current_user.books.exists?(book.id)
+  end
 end

app/views/books/index.html.erb

@@ -1,9 +1,9 @@
 <div class='container'>
   <div class='row'>
-    <div class='col s2'>Title</div>
-    <div class='col s3'>Authors</div>
-    <div class='col s1'>Price</div>
-    <div class='col s3'>Quantity</div>
+    <h6 class='col s2'>Title</h6>
+    <h6 class='col s3'>Authors</h6>
+    <h6 class='col s1'>Price</h6>
+    <h6 class='col s3'>Quantity</h6>
 
   </div>
   <% @books.each do |book| %>
@@ -24,7 +24,7 @@
         <%= book.quantity %>
       </div>
       <div class='col s2'>
-        <% if logged_in? && book.quantity.positive? && !current_user.books.exists?(book.id) %>
+        <% if can_book_be_added?(book) %>
           <%= link_to 'Add', "book/#{book.id}/add_to_cart", method: :post, class: "btn" %>
         <% end %>
       </div>

app/views/books/shopping_cart.erb

@@ -1,8 +1,8 @@
 <div class='container'>
   <div class='row'>
-    <div class='col s2'>Title</div>
-    <div class='col s3'>Authors</div>
-    <div class='col s1'>Price</div>
+    <h6 class='col s2'>Title</h6>
+    <h6 class='col s3'>Authors</h6>
+    <h6 class='col s1'>Price</h6>
   </div>
   <% @books.each do |book| %>
     <div class='row'>
@@ -19,4 +19,8 @@
       </div>
     </div>
   <% end %>
+  <div class='row'>
+    <div class='col s5'><strong>Summary</strong></div>
+    <div class='col s1'><%= cart_summary %></div>
+  </div>
 </div>