ActiveAdminでメニューの並び順を1箇所で管理する

MENU_ITEMS = [
  [:item, 'Dashboard', {label: proc{ I18n.t("active_admin.dashboard") }}],
  [:item, 'Item', {}],
  [:item, 'Skill', {label: '特技検索'}],
  [:item, 'Evolution', {label: '進化検索'}],
  [:category, 'master',
    [
      ['Element', {}],
      ['BoxKind', {}],
    ]
  ],
  [:item, 'Comment', {}],
]

MENU_ITEM_EXCLUDES = %w()

def inverted_menu_items
  MENU_ITEMS.each_with_object([]).with_index do |(source_item, inverted), index|
    case source_item.first
    when :item
      _, page, args = source_item
      inverted << [page, args.merge(priority: index)]
    when :category
      _, category, children = source_item
      inverted << [category, {label: I18n.t("active_admin.menu.#{category}"), priority: index}]
      children.each_with_index do |category_item, index2|
        page, args = category_item
        inverted << [page, args.merge(parent: I18n.t("active_admin.menu.#{category}"), priority: index2)]
      end
    end
  end
end

def check_index_and_add_priority_and(name, &block)
  index = inverted_menu_items.map(&:first).index(name)
  if index
    args = inverted_menu_items[index][1]
    block.call(args)
  elsif !(MENU_ITEM_EXCLUDES.include? name)
    raise "#{name} is NOT placed into the menu"
  end
end

ActiveAdmin.application.namespace(false).build_menu do |menu|
  MENU_ITEMS.select {|i| i.first == :category}.each do |item|
    name = item[1]
    check_index_and_add_priority_and(name) do |args|
      menu.add args
    end
  end
end

class ActiveAdmin::DSL
  def allocate_to_menu
    name = config.resource_name.name
    check_index_and_add_priority_and(name) do |args|
      menu args
    end
  end
end

module MenuAllocator
  def register(*args, &block)
    super *args do
      allocate_to_menu
      self.instance_eval &block
    end
  end

  def register_page(*args, &block)
    super *args do
      allocate_to_menu
      self.instance_eval &block
    end
  end
end

module ActiveAdmin
  class Namespace
    prepend MenuAllocator
  end
end