1

一段时间以来，我一直在为一个 Rails 问题绞尽脑汁，想验证我的发现。我试图让 Has_and_belongs_to_many 关系正常工作，但无法连接我的两个类，auctionItem 和 category。首先，这是我的迁移文件和解决问题之前的两个类：

迁移文件：

class AuctionItemsCategories < ActiveRecord::Migration
  def up
    create_table 'auction_items_categories', :id=>false do |t|
      t.reference :auctionItem_id
      t.references :category_id 
    end
  end

  def down
    drop_table 'auction_items_categories'
  end
end

类别.rb

class Category < ActiveRecord::Base
  has_and_belongs_to_many :auction_items
end

拍卖物品.rb

class AuctionItem < ActiveRecord::Base
  has_and_belongs_to_many :categories
end

创建 AuctionItem 的实例后，我尝试了

auction_item = AuctionItem.last
auction_item.categories

...并得到以下错误：

NoMethodError: undefined method `categories' for #<AuctionItem:0x0000010521b870>

经过一番研究，我发现将特定类添加到 has_and_belongs_to_many 有帮助：

类别模型

has_and_belongs_to_many :auction_items , :class_name => 'AuctionItem'

auction_item 模型

has_and_belongs_to_many :categories , :class_name => 'Category'

这解决了这个问题，我能够访问类别表。我继续尝试在拍卖品中添加一个类别：

auction_item.categories << category

然后我收到以下错误：

SELECT "auction_items".* FROM "auction_items" INNER JOIN "auction_items_categories" ON "auction_items"."id" = "auction_items_categories"."auction_item_id" WHERE "auction_items_categories"."category_id" = 2
SQLite3::SQLException: no such column: auction_items_categories.auction_item_id: SELECT "auction_items".* FROM "auction_items" INNER JOIN "auction_items_categories" ON "auction_items"."id" = "auction_items_categories"."auction_item_id" WHERE "auction_items_categories"."category_id" = 2
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: auction_items_categories.auction_item_id: SELECT "auction_items".* FROM "auction_items" INNER JOIN "auction_items_categories" ON "auction_items"."id" = "auction_items_categories"."auction_item_id" WHERE "auction_items_categories"."category_id" = 2

如果您注意到，查询正在尝试获取auction_item.id 而不是AuctionItem.id。为了使连接正常工作，我必须将迁移文件更改为以下内容：

class AuctionItemsCategories < ActiveRecord::Migration
  def up
    create_table 'auction_items_categories', :id=>false do |t|
      t.integer :auction_item_id
      t.integer :category_id 
    end
  end

  def down
    drop_table 'auction_items_categories'
  end
end

长话短说/TL DR 版本：对我来说，似乎当用多个单词命名你的类并使用驼峰式大小写时，如果它有一个下划线，rails 不会将你的复数类名单数化回它的原始状态。例如，我的类名是 AuctionItem ，它变成了模型的auction_items。不是搜索auctionitem.id，而是查找的sql 调用是auction_item.id，它是auction_items 的单数化版本。为什么不搜索auctionitem.id？以后在制作多词类的关联表时，是否使用单数下划线 id 版本的模型名称？

4

1 回答 1

1

您的原始迁移不正确。它应该如下所示：

class AuctionItemsCategories < ActiveRecord::Migration    
  def up
    create_table 'auction_items_categories', :id => false do |t|
      t.references :auction_item
      t.references :category
    end
  end

  def down
    drop_table 'auction_items_categories'
  end
end

使用参考时应指定符号化模型名称。

于 2012-12-14T21:31:42.890 回答