Multiple belongs_to to the same table
model-associations, ruby-on-rails, ruby-on-rails-3
Solution
In your two `belongs_to` methods, change the `foreign_key` option to `primary_key`, leaving everything else as is.
belongs_to :top_currency, :class_name => 'Currency', :primary_key => 'top_currency_id'
# ...
By default, an associated object's primary key is `id`. However, your currency model has three primary keys, the expected `id` plus two extra keys: `top_currency_id` and `bottom_currency_id`. Active Record needs to know which key to look for. Tell it with the `primary_key` option.
The `foreign_key` option is needed when a foreign key is different than the association's name (`belongs_to :name`) plus "`_id`". Since your foreign key matches the association name plus "`_id`," you do not need to use the `foreign_key` option.
Problem
I have two tables: currencies and rates ``` currencies: id:int, code:string, name: string rates: id:int, top_currency_id:int, bottom_currency_id:int, rate:float ``` And I have two active records for them: ``` 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 ``` So the problem is: When I'm tring to execute following code: top_currency = Currency.find_by_id(1) @test = Rate.where(:top_currency=>top_currency) I getting following error: ``` Mysql2::Error: Unknown column 'rates.top_currency' in 'where clause': SELECT `rates`.* FROM `rates` WHERE `rates`.`top_currency` = 1 ``` Why Rails's magic doesn't work? Many thanks.