Question

The rails docs are apparently in error – http://guides.rubyonrails.org/association_basics.html#selfjoins)

In designing a data model, you will sometimes find a model that should
have a relation to itself. […]

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

With this setup, you can retrieve @employee.subordinates and
@employee.manager.

---

Actually, in at least the console, an error is generated in the above if foreign_key is not “employee_id”.

Here’s my specific code:

#Table name: plates
#
#  id         :integer         not null, primary key
#  name       :string(255)
#  datetime   :datetime
#  parent_id  :integer
#  precision  :integer
#  tags       :string(255)
#  created_at :datetime        not null
#  updated_at :datetime        not null
#

class Plate < ActiveRecord::Base

  has_many :templates

  has_many :children, :class_name => "Plate"
  belongs_to :parent, :class_name => "Plate",
    :foreign_key => "parent_id"
  [...] 

…and the query I run:

irb(main):002:0> Plate.find_by_name("blog090822").children.first

If I run that it generates SQL looking for plate_id and then returns an error for non-existant column. If I change the column name to plate_id through a migration, reseed the DB and rerun the query it works.

If this is a rails documentation error, how commonplace is that.

Answer 1

In the API documents, there are some explanations to foreign_key. For belongs_to:

By default this is guessed to be the name of the association with an “_id” suffix.

For has_many:

By default this is guessed to be the name of this class in lower-case and “_id” suffixed.

So in your case, the foreign_key for :children should be plate_id, and the foreign_key for :parent should be parent_id.
To let your code work while keeping the data schema, only the foreign_key for :children is necessary, which override the default foreign_key with parent_id.

The code in the Rails guide could be wrong.