如何在Rails 3的下拉列表中安排Ancestry树中的条目？

gem 'ancestry'

class Category< ActiveRecord::Base
  has_ancestry
end

module CategoriesHelper
  def ancestry_select(categories, select_options = {})
    select_options.reverse_merge!({
      selected: nil,
      disabled: nil,
      include_blank: true,
      prompt: "Select a category"
    })

    content_tag(:select, name: "category[parent_id]") do
      options_for_select(
        nested_options_for_select(categories, select_options),
        select_options[:selected]
      )
    end
  end

  private

  def nested_options_for_select(categories, select_options, level = 0)
    result = []

    categories.each do |category|
      option = [("-" * level) + category.name, category.id]
      if select_options[:disabled] && select_options[:disabled].include?(category)
        option[0] = "[#{option[0]}]"
      end
      result<< option

      if category.children.any?
        result += nested_options_for_select(category.children, select_options, level + 1)
      end
    end

    result
  end
end

<%= form_for(@category) do |f| %>
  <div class="field">
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </div>

  <div class="field">
    <%= f.label :parent_id %><br />
    <%= ancestry_select(Category.arrange) %>
  </div>

  <div class="actions">
    <%= f.submit "Save" %>
  </div>
<% end %>