Question

I feel the following self join model example given on http://guides.rubyonrails.org/association_basics.html#self-joins is incorrect.

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

I feel it should be as following. Can you please suggest which one is correct and why?

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

My rationale: The model bearing belongs_to relation carries the foreign_key for the model it references to.

Answer 1

The example is correct.

The “convention over configuration” mantra applies here, you only need to specify what the foreign key if it is not "#{name_of_association}_id"

Therefore belongs_to :manager, :class_name => "Employee" implies the foreign key is manager_id

However has_many :subordinates, :class_name => "Employee" assumes subordinates_id to be the childrens’ foreign key, which is why if must be defined. The definition of :foreign_key in a has_many will be for the child and will join against the current model’s id.