You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: 044_menu_update_users.rb
    path of file: ./repos/expertiza/db/migrate
    
    the code of the file until where you have to start completion: class MenuUpdateUsers < ActiveRecord::Migration[4.2]
  def self.up
    permission = Permission.find_by_name
    
  def self.down
    site_controller = SiteController.find_by_name('users')
    unless site_controller.nil?
      action = ControllerAction.where(['site_controller_id = ? and name = ?', site_controller.id, 'list']).first
      unless action.nil?
        action.permission_id = nil
        action.save
      end