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.

Philip Hallstrom · Accepted Answer · 2015-08-12T20:03:00.263

12

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

edited Aug 12 '15 at 20:03

answered Aug 12 '15 at 18:48

Philip Hallstrom

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Hum. True. That's what OP asked. – Arup Rakshit Aug 12 '15 at 18:53
I am not sure that is what I am looking for. I am new to Ruby, so I might be wrong. But basically, I have an Employee model with a self-join. I want the foreign-key manager_id to be set to null for employees when the corresponding manager (also an entity of the employee table) is deleted. What you seem to be suggesting is a different Manager class itself – Neel Vasa Aug 12 '15 at 19:04
@NeelVasa Oops! My bad. I missed that part. Same idea though. Add the `dependent: :nullify` to your `has_many :subordinates` line. I'll update the code. – Philip Hallstrom Aug 12 '15 at 20:02

score 4 · Answer 2 · answered Jun 01 '18 at 15:20

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}

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

2 Answers2