使用build_menu命令在Active Admin中获取NoMethodError

Question

我正在自定义Active Admin中的菜单。根据此处http://activeadmin.info/docs/2-resource-customization.html#customize_the_menu的文档，我将此代码放在active_admin.rb初始化文件中：

  config.namespace :admin do |admin|
     admin.build_menu do |menu|
       menu.add :label => "Churches", :url => "/", :priority => 0
       menu.add :label => "Liaisons", :url => "/", :priority => 0

       menu.add :label => "Reports" do |reports|
         reports.add :label => "Google", :url => "http://google.com", :html_options => { :target => :blank }
         reports.add :label => "Facebook", :url => "http://facebook.com"
         reports.add :label => "Github", :url => "http://github.com"
       end
     end
  end

不幸的是，这会在应用程序初始化时产生以下错误：在block (2 levels) in <top (required)>': undefined method build_menu'中，对于ActiveAdmin :: Namespace：0x007fd241803458＆gt; （NoMethodError）

有谁能帮我看看我在这里做错了什么？