Rails migration set null to false with no default

migration.rb

class AddPrivacyToSubscription < ActiveRecord::Migration
  def change
    add_column :subscriptions, :privacy, :boolean, :default => false, :null => false
    change_column_default(:subscriptions, :privacy, nil) 
  end
end

I didn't know change_column_default method. It's indeed very useful when you are introducing a new field on a populated table and you don't' want to set a default value for the new records to come.