Multiple belongs_to to the same table - ruby-on-rails

Question

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.

Answer 1

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.

Answer 2

From what I see, your code should work in theory. But I do think you are being a bit redundant.
It should be enough to just do this:
class Rate < ActiveRecord::Base
belongs_to :top_currency, class_name: 'Currency'
belongs_to :bottom_currency, class_name: 'Currency'
end
Rails will infer that the foreign key for top_currency is top_currency_id, and bottom_currency_id for bottom_currency.

Answer 3

I don't think you can query on the relationship like that. To use your example:
top_currency = Currency.find_by_id(1)
#test = Rate.where(:top_currency=>top_currency)
You'd have to change it to this:
top_currency = Currency.find_by_id(1)
#test = Rate.where(:top_currency_id => top_currency.id)
But it might just be easier to do this:
top_currency = Currency.find_by_id(1)
#test = top_currency.rates