最近尝试了下 mongoid-tree，觉得挺好用，可以实现无限级分类，顺便写了一个 helper，来帮助在页面展示分类的下拉菜单，看代码： category.rb

#coding:utf-8
class Category
  include Mongoid::Document
  include Mongoid::Tree

  field :name, type: String
  field :state, type: Integer, default: 0
  field :content, type: String


  def get_tree(tree)
    if self.root?
      tree << ["#{self.name}","#{self.id}"]
    else
      level = self.depth  #level表示当前分类是第几级分类(0代表顶级，依次类推)
      bloc = "　" * level  #这里的空格是英文全角空格
      tree << [ bloc + "|--#{self.name}","#{self.id}"]
    end

    if self.children?
      self.children.each do |chi|
        chi.get_tree(tree)  
      end
    end

  end  

end

application_helper.rb

module ApplicationHelper
  def category_tree
    tree = []
    Category.roots.each do |cate|
      cate.get_tree(tree)
    end
    return tree
  end
end

_form.html.erb

<div class="control-group">
  <%= f.label :parent_id, :class => 'control-label' %>
  <div class="controls">
    <%= f.select :parent_id,category_tree,{:include_blank => '顶级分类'}%>
  </div>
</div>

效果图如下：