受不了啦，给我一个理由阿： M 如下：

class Trip
  include Mongoid::Document
  include Mongoid::Timestamps
  field :title, :type => String
  has_many :steps, :dependent => :destroy
  accepts_nested_attributes_for :steps
end

class Step
  include Mongoid::Document
  include Mongoid::Timestamps
  field :content, :type => String
  validates_presence_of :content
  belongs_to :trip
end

C 如下：

class TripsController < ApplicationController
  def new
    @trip = Trip.new
    respond_with(@trip) do |format|
      format.html # new.html.erb
      format.json { render json: trip }
    end
  end
  ......

V 如下：(haml)

= simple_form_for @trip do |f|
  = f.input :title
  = f.simple_fields_for @trip.steps.build do |sf|
    = sf.input :content
  = f.submit 'Save'

不知为何，form 页面看上去很正常：

<input class="string required" id="trip_step_content" name="trip[step][content]" required="required" size="50" type="text">

但 post 上去，params 就变成了：

Parameters: {"utf8"=>"✓", "authenticity_token"=>"GgiUuDS8YcBmFhkt5jYGNXGBho/eVZuVQhB4K1S0KEo=", "trip"=>{"title"=>"111", "step"=>{"content"=>"111"}}, "commit"=>"Save"}

而 mongoid 要求的 step 参数却是：steps_attributes; 但凡能 stackoverflow 到的，都不会这样阿。 求教：mongoid 的 accepts_nested_attributes_for 应该如何写 viewer 阿； simple_field_for 如何写？