Custom foreign keys in ruby on rails

Posted on 21 June 2006

Custom foreign keys in Rails are handy if you have a legacy database where you cannot change table/field names.

In my case it wasn't to do with a legacy db; I wanted to create a foreign key that pointed to the same table as another foreign key.

The best way to explain this is by example. I have a table that stores data relating to swaps. Each swap relates to two adverts. So I needed two foreign keys for every swap, one each for the two adverts.

So in my swaps table I have fields:

offered_advert_id 
advert_id

In my swap model:

class Swap < ActiveRecord::Base
 belongs_to :advert
 belongs_to :offered_advert, :class_name => "Advert", :foreign_key => 'offered_advert_id'
end

In my advert model:

class Advert < ActiveRecord::Base
 has_many :swaps
 has_many :offered_swaps, :class_name => 'Swap', :foreign_key => 'offered_advert_id'
end

advert_id follows the typical rails assumptions and therefore the relationship between a swap and the advert_id is as expected.

The complication is with the offered_advert_id field which needs the additional relationship entered into both models.

Essentially it doesn't matter what you call the :has_many or the :belongs_to but you need to be specific about the :class_name => which must relate to the two models involved and the :foreign_key => must point to the field you want the relationship to use.

Using the above code gave me the method such as:

swap.offered_advert

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