CartItem クラス追加

depot/app/models/cart_item.rb

@@ -0,0 +1,20 @@
+class CartItem
+    attr_reader :product, :quantity
+
+    def initialize (product)
+        @product = product
+        @quantity = 1
+    end
+
+    def increment_quantity
+        @quantity += 1
+    end
+
+    def title
+        @product.title
+    end
+
+    def price
+        @product.price * @quantity
+    end
+end