mastodon/db/migrate/20181219235220_add_created_by_application_id_to_users.rb

class AddCreatedByApplicationIdToUsers < ActiveRecord::Migration[5.2]
  disable_ddl_transaction!

  def change
    add_reference :users, :created_by_application, foreign_key: { to_table: 'oauth_applications', on_delete: :nullify }, index: false
    add_index :users, :created_by_application_id, algorithm: :concurrently
  end
end