多个belongs_to到同一个表

Question

我有两张桌子:

货币和利率

currencies: id:int, code:string, name: string

rates: id:int, top_currency_id:int, bottom_currency_id:int, rate:float

我有两个活跃的记录:

class Rate < ActiveRecord::Base
  attr_accessible :bottom_currency, :rate, :top_currency, :top_currency_id

  belongs_to :top_currency, :class_name => 'Currency', :foreign_key => 'top_currency_id'
  belongs_to :bottom_currency, :class_name => 'Currency', :foreign_key => 'bottom_currency_id'
end


class Currency < ActiveRecord::Base
  attr_accessible :code, :name

  has_many :rates
end

所以问题是:当我要执行以下代码时:top_currency = Currency.find_by_id(1)@test = Rate.where(:top_currency => top_currency)

我收到以下错误:

Mysql2::Error: Unknown column 'rates.top_currency' in 
'where clause': SELECT `rates`.* FROM `rates`  WHERE `rates`.`top_currency` = 1

为什么Rails的魔法不起作用？

非常感谢.

Answer 1

据我所知，您的代码理论上应该可以工作。但我确实认为你有点多余。

这样做就足够了：

class Rate < ActiveRecord::Base
  belongs_to :top_currency, class_name: 'Currency'
  belongs_to :bottom_currency, class_name: 'Currency'
end

Rails会推断为外键top_currency是top_currency_id，和bottom_currency_id为bottom_currency。

Answer 2

在您的两种belongs_to方法中,将foreign_key选项更改为primary_key,将其他所有内容保留为原样.

belongs_to :top_currency, :class_name => 'Currency', :primary_key => 'top_currency_id'
# ...

默认情况下,关联对象的主键是id.但是,您的货币模型有三个主键,预期id加上两个额外的键:top_currency_id和bottom_currency_id.Active Record需要知道要查找的密钥.用primary_key选项告诉它.

foreign_key当外键不同于关联的名称(belongs_to :name)加上" _id" 时,需要该选项.由于您的外键与关联名称加上" _id," 匹配,因此您无需使用该foreign_key选项.