cart.rb

class Cart < ActiveRecord::Base
  has_many :cart_items, :dependent => :destroy
  has_many :items, :through => :cart_items, :source => :product
  def add_product_to_cart(product)
    items << product
  end
  def update_quantity_to_cart(product_id, quantity, current_cart_id)
    ## do something..
  end  
  
  # diff
  #def total_price(cart_id)
  def total_price #不需要添加(cart_id)
    
    #找出這台車有的產品及產品價格(test)
    #items.each(){ |item| item.price}
    #找出數量(test)
    #artItem.where(product_id: 2,cart_id: 5).take!.quantity
    #運算
    
    #diff
    #items.inject(0){|sum, item| sum + item.price * CartItem.where("product_id = ? AND cart_id = ?", item.id, cart_id).take!.quantity }
    items.inject(0) { |sum, item| sum + (item.price * self.cart_items.find_by(:product_id => item.id).quantity) } # 因為此為 current_cart 的方法，所以不用再利用 cart_id 了，這樣會更方便
    #note利用.map & 把quantity拆成另一個method
  end
end

carts_helper.rb

module CartsHelper
  def show_cart_total_price(current_cart)
    #CartItem的quantity*Product的price
    
    # diff
    #current_cart.total_price(current_cart.id)
    current_cart.total_price #因為已經擁有current_cart了所以就不用再多放 current_cart.id 進去～這是一樣的
  end
end