Ruby on Rails guides document on Activerecord association incorrect?

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.