Rails join(includes) tables where “id” is not present in one table and different primary key

Question

I have two models where one has "id" as the primary key. Second model doesn't have "id" as primary key. Instead has a composite key. How to join these?

class A < ActiveRecord::Base
  has_one :B, :order => "dt DESC"
end

class B < ActiveRecord::Base
  belongs_to :A, :foreign_key => :emp_id
end

The emp_id acts as the foreign key. In the controller for A,

arel = A.includes(:b)

This fails. I get following error,

ActiveRecord::StatementInvalid (Mysql2::Error: Unknown column 'b.id' in 'field list': SELECT a . id AS t0_r0,....

How shall I proceed?

Answer 1

Try setting the primary_key on the belongs_to or the has_one .

More info on these options can be found at,

ActiveRecord belongs_to

ActiveRecord has_one