Question

Wow I’ve been struggling with this for whole day, following the “official” ruby on rails guides document, and just discovered that I might have been misguided by the document all along. I just want to confirm if this is true.

If you go to http://guides.rubyonrails.org/association_basics.html and under 2.10. self joins section it says:

class Employee < ActiveRecord::Base
  has_many :subordinates, :class_name => "Employee"
  belongs_to :manager, :class_name => "Employee",
    :foreign_key => "manager_id"
end

Now, I’m a newbie and just believed in this code (What else can I do?) and wrote some code that’s a variation of this self join case. However the more I looked at it the more it didn’t feel right. isn’t :subordinates supposed to have the :foreign_key field instead of :manager? Anyway I’ve just changed it so that the code is something like:

class Employee < ActiveRecord::Base
  has_many :subordinates, :class_name => "Employee", :foreign_key => "manager_id"
  belongs_to :manager, :class_name => "Employee"
end

and now it’s working. Am I missing something? Or is the official document wrong? It’s hard to believe that the official document would present incorrect information but maybe that’s the way it is.

Answer 1

That’s right, the guide document is incorrect at the time of this writing.

The belongs_to doesn’t need the :foreign_key option because AR will infer manager_id from the name of the association (“manager”). As documented, AR would raise an error when, given an Employee @dwight one attempts to @dwight.subordinates, because AR would use employee_id in the WHERE condition of the SELECT statement.

According to the AR documentation passing the :foreign_key option to has_many results in declaring the FK that will be used when generating the query for @dwight.subordinates.