ActiveRecord: Nullify foreign key after deleting a child in self-joins

Question

I have a self-join for my model in ActiveRecord, as follows:

class Employee < ActiveRecord::Base
  has_many :subordinates, class_name: "Employee",
                          foreign_key: "manager_id"

  belongs_to :manager, class_name: "Employee"
end 

If I delete a Manager row, I expect 'manager_id' foreign key values for all employees that were under that Manager to be set to NULL. Is this something that is handles implicitly by ActiveRecord, or is it something that needs to be defined somewhere.

Answer 1

You want to add dependent: :nullify to your has_many association.

class Employee…
  has_many :subordinates, class_name: "Employee",
                          foreign_key: "manager_id",
                          dependent: :nullify
end

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_many

Answer 2

Ideally you should have the trigger be set in your database and not rely on Active Record to perform updates.

---

If you are just creating the table, you can:

create_table :employees do |t|
  t.references :manager, foreign_key: {to_table: :employees, on_delete: :nullify}
  # ...
end

---

Alternatively, if the table already exists and you are just adding the reference:

add_reference :employees,
              :manager,
              foreign_key: {to_table: :employees, on_delete: :nullify}