ActiveRecord has_many メモ

rails 
create_table :reservations, :force => "true" do |t|
  t.column "name", :string
  r.column "lock_versioin", :integer
end
create_table :units, :force => "true" do |t|
  t.column "name", :string
  t.column "reservation_id", :integer
  r.column "lock_versioin", :integer
end
class Reservation < ActiveRecord::Base
  has_many :units ,
           :dependent => nullify
end
class Unit < ActiveRecord::Base
  belongs_to :reservation
end

で

r = Reservation.find(1)
r.units.size
r.units[0].id
=> 1
r.units[1].id
=> 2

のときに

r.destroy
Unit.find(1).reservation_id
=> nil
Unit.find(2).reservation_id
=> nil

だが

r.units.delete(u1)

でdeleteじゃなくてnullifyでよろしくおねがいしたい