ROR关联的语法

Question

事先为另一个新手问题表示歉意，但是ROR语法只是没有让我满意，我无法理解这些快捷方式和约定（尽管已经读了几本书！）-

我有效地从一本书中复制了此内容，但是我正在尝试确定什么是构建，创建等？

@cart = current_cart
product = Catalog::Product.find(params[:product_id])
Rails.logger.debug { "Value in cart id " + @cart.id.to_s }

@checkout_line_item = @cart.line_items.build(product: product)

respond_to do |format|
  if @checkout_line_item.save...

日志的输出是这样的：

Processing by Checkout::LineItemsController#create as HTML
Parameters: {"utf8"=>"✓",  "authenticity_token"=>"9NH+xgDPTf/iN7RCdPd8H9rAIqWsSVB/f/rIT++Kk7M=", "product_id"=>"7"}
Created a line item
(0.1ms)  BEGIN
SQL (2.0ms)  INSERT INTO `checkout_carts` (`created_at`, `discounts`, `grand_total`,   `loyalty_points`, `order_date`, `subtotal`, `timestamps`, `total_tax`, `updated_at`) VALUES    ('2012-08-21 11:06:15', NULL, NULL, NULL, NULL, NULL, NULL, NULL, '2012-08-21 11:06:15')
(0.2ms)  COMMIT
Catalog::Product Load (0.2ms)  SELECT `products`.* FROM `products` WHERE `products`.`id` =   7 LIMIT 1
Value in cart id 8
Completed 500 Internal Server Error in 5ms

NoMethodError (undefined method `save' for nil:NilClass):
app/controllers/checkout/line_items_controller.rb:55:in `block in create'
app/controllers/checkout/line_items_controller.rb:54:in `create'

我猜问题出在构建语法上，它在其中构建结帐订单项，或者可能我将has_many关联设置错误。 这足以帮助我进行故障排除吗？ 还是应该发布模型声明？

型号更新：

class Checkout::LineItem < ActiveRecord::Base
  attr_accessible :customer_update_date, :inventory_status, :line_item_color, :line_item_description, :line_item_size, :line_item_tagline, :line_item_total, :quantity, :sku_id, :style_id, :tax, :tax_code, :timestamps, :unit_price, :product
  belongs_to :cart
  belongs_to :product, :class_name => 'Catalog::Product'
end

class Checkout::Cart < ActiveRecord::Base
  attr_accessible :discounts, :grand_total, :loyalty_points, :order_date, :subtotal, :timestamps, :total_tax

  has_many :line_items, dependent: :destroy
end

module Catalog
  class Product < ActiveRecord::Base
    attr_accessible :assoc_product,:product_id, :merch_associations, :aux_description, :buyable, :long_description, :name, :on_special, :part_number, :release_date, :short_description, :withdraw_date, :occasion
   <<clipped for brevity>>

has_many :line_items, :class_name => 'Checkout::LineItem'
...
  end

无法回答我自己的问题，但我认为我得到了答案：

看来我需要将购物车添加到构建调用中...

这似乎奏效了（我认为，还有另一个阻塞问题，但是我可以对它进行排序）：

@cart = current_cart
product = Catalog::Product.find(params[:product_id])
Rails.logger.debug { "Value in cart id " + @cart.id.to_s }

@checkout_line_item = @cart.line_items.build(product: product, cart: @cart)

Answer 1

Build实质上是为空白对象创建并保留空间。 这使您可以创建关联对象而无需保存它。 我经历过的唯一用例是一个嵌套表单，其中一个事件有多个日期，因此我使用5.times.build创建了5个空日期关联。