需求是这样的。 我需要储存一些 metadata，所以搞了个序列化的 information 作为储存。然后呢，为了用着方便，通过 method_missing 完成了幽灵方法。这样我就可以通过类似 s.info_location 来访问 s.infomation['location']，同时也方便在 form_for 中使用。 代码如下，其余代码均由 scaffold 生成：

class Series < ActiveRecord::Base
  validates :name, :uniqueness => true
  serialize :information, JSON
  def method_missing(name,* args)
    super unless respond_to? name
    attribute = name.to_s
    attribute = attribute[5..attribute.length]
    if attribute[-1]=='='
      self.information[attribute.chop]=args[0]
    else
      self.information[attribute]
    end
  end
   def respond_to?(method)
     method_str= method.to_s
     method_str.include? 'info_' || super
   end
  def to_s
    name
  end
end

结果出现的情况是，渲染/series/2/edit 页面时，却错误的渲染成了 new 的页面，如下图所示：

将 Series.rb 修改为下面代码后，正常渲染。（既，删掉对 respond_to? 的复写）

class Series < ActiveRecord::Base
  has_many :episodes
  has_many :hobbies
  validates :name, :uniqueness => true
  serialize :information, JSON
  def method_missing(name,* args)
    attribute = name.to_s
    super unless attribute.include? 'info_'
    attribute = attribute[5..attribute.length]
    if attribute[-1]=='='
      self.information[attribute.chop]=args[0]
    else
      self.information[attribute]
    end
  end
  # def respond_to?(method)
  #   method_str= method.to_s
  #   method_str.include? 'info_' || super
  # end
  def to_s
    name
  end
end

最后完成效果是： views/series/_form.html.erb

<%= form_for(@series) do |f| %>
  <% if @series.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(@series.errors.count, "error") %> prohibited this series from being saved:</h2>

      <ul>
      <% @series.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= f.label :name %><br>
    <%= f.text_field :name %>
  </div>
  <div class="field">
    <%= f.label :description %><br>
    <%= f.text_field :description %>
  </div>
  <div class="field">
    <%= f.label :info_location %><br>
    <%= f.text_field :info_location %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>
<% end %>

controllers/series_controller.rb

class SeriesController < ApplicationController
  before_action :set_series, only: [:show, :edit, :update, :destroy]

  def index
    @series = Series.all
  end

  def show
  end

  def new
    @series = Series.new
  end

  def edit
  end

  def create
    @series = Series.new(series_params)

    respond_to do |format|
      if @series.save
        format.html { redirect_to @series, notice: 'Series was successfully created.' }
        format.json { render :show, status: :created, location: @series }
      else
        format.html { render :new }
        format.json { render json: @series.errors, status: :unprocessable_entity }
      end
    end
  end

  def update
    respond_to do |format|
      if @series.update(series_params)
        format.html { redirect_to @series, notice: 'Series was successfully updated.' }
        format.json { render :show, status: :ok, location: @series }
      else
        format.html { render :edit }
        format.json { render json: @series.errors, status: :unprocessable_entity }
      end
    end
  end

  def destroy
    @series.destroy
    respond_to do |format|
      format.html { redirect_to series_index_url, notice: 'Series was successfully destroyed.' }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_series
      @series = Series.find(params[:id])
    end

    # Never trust parameters from the scary internet, only allow the white list through.
   ######################################
   #  add :info_xxx to access information['xxx']               # 
   ######################################
    def series_params
      params.require(:series).permit(:name, :description, :info_location)
    end
end

既，在 form_for 中可以直接像普通属性一样通过 info_xxx 来访问 information 这个 hash 中的变量。