我做了以下事情：

form do |f|
  f.inputs "Details" do
    f.input :instructor,:as => :select 
    f.input :title
    f.input :name
    f.input :price
    f.input :discount
    f.input :slug
    f.inputs "Sections" do
       f.has_many :sections,:header=>"" do |section|
         section.input :name
         section.input :position
         if section.object.id
           section.input :_destroy,:as=>:boolean,:required => false,:label=>'Remove'
         end
 
         section.has_many :lessons,:header=>"Lessons" do |lesson|
           lesson.input :title
           lesson.input :position
           lesson.input :duration
           lesson.input :_destroy,:label=>'Remove'
         end
       end
   end
 
  end
  f.buttons
end

我的模型如下：

class Course < ActiveRecord::Base
    has_many :sections,:dependent => :delete_all 
    accepts_nested_attributes_for :sections,:allow_destroy => true
    attr_accessible :sections_attributes
 ....
 
class Section < ActiveRecord::Base
    belongs_to :course
    has_many :lessons,:dependent => :delete_all
    attr_accessible :course_id,:name,:position
    accepts_nested_attributes_for :lessons,:allow_destroy => true
    attr_accessible :lessons_attributes
....
 
class Lesson < ActiveRecord::Base
    belongs_to :section
    attr_accessible :duration,:position,:section_id,:title
....

而且效果很棒！我不知道如果我更深层次会发生什么.