has_one：through - build_association undefined？

Question

我有一个部分模型，其中一个部分可以是另一个部分（子部分）的父级。

这里是我的模型：

class Section < ActiveRecord::Base 
    has_many :exercises 

    has_one :parent_link, 
    :foreign_key => 'subsection_id', 
    :class_name => 'SectionLink', 
    :dependent => :destroy 

    has_one :parent, :through => :parent_link 

    has_many :subsection_links, 
    :foreign_key => 'parent_id', 
    :class_name => 'SectionLink', 
    :dependent => :destroy 

    has_many :subsections, :through => :subsection_links 

    attr_accessor :parent_id 

    def to_param 
    "#{id}-#{description.parameterize}" 
    end 

    def self.search(search) 
    if search 
     find(:all, :conditions => ['description LIKE ?', "%#{search}%"]) 
    else 
     find(:all) 
    end 
    end 
end 

而且关联模型：

class SectionLink < ActiveRecord::Base 
    belongs_to :parent, :class_name => 'Section' 
    belongs_to :subsection, :class_name => 'Section' 
end 

我的控制器：

class SectionsController < ApplicationController 
    # GET /sections 
    # GET /sections.json 
    def index 
    @sections = Section.order("subsections_count DESC").search(params[:search]) 

    respond_to do |format| 
     format.html # index.html.erb 
    end 
    end 

    # GET /sections/1 
    # GET /sections/1.json 
    def show 
    @section = Section.find(params[:id]) 
    @subsections = @section.subsections 
    @exercises = @section.exercises 

    respond_to do |format| 
     format.html # show.html.erb 
     format.json { render json: @section } 
    end 
    end 

    # GET /sections/new 
    # GET /sections/new.json 
    def new 
    @section = Section.new 
    @section.parent_id = params[:parent] 

    respond_to do |format| 
     format.html # new.html.erb 
     format.json { render json: @section } 
    end 
    end 

    # GET /sections/1/edit 
    def edit 
    @section = Section.find(params[:id]) 
    end 

    # POST /sections 
    # POST /sections.json 
    def create 
    @section = Section.new(params[:section]) 
    @parent = @section.build_parent(:parent_id => @section.parent_id) unless @section.parent_id.empty? 

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

    # PUT /sections/1 
    # PUT /sections/1.json 
    def update 
    @section = Section.find(params[:id]) 

    respond_to do |format| 
     if @section.update_attributes(params[:section]) 
     format.html { redirect_to @section, notice: 'Section was successfully updated.' } 
     format.json { head :no_content } 
     else 
     format.html { render action: "edit" } 
     format.json { render json: @section.errors, status: :unprocessable_entity } 
     end 
    end 
    end 

    # DELETE /sections/1 
    # DELETE /sections/1.json 
    def destroy 
    @section = Section.find(params[:id]) 
    @section.destroy 

    respond_to do |format| 
     format.html { redirect_to sections_url } 
     format.json { head :no_content } 
    end 
    end 
end 

父ID通过一个隐藏字段在美联储形式：

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

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

    <div class="field"> 
    <%= f.label :description %><br /> 
    <%= f.text_field :description %> 
    </div> 
    <div class="field"> 
    <%= f.label :body %><br /> 
    <%= f.text_area :body %> 
    </div> 
    <%= f.hidden_field :parent_id, :value => @section.parent_id %> 
    <div class="actions"> 
    <%= f.submit %> 
    </div> 
<% end %> 

我越来越

undefined method 'build_parent' for #<Section:0xb4f1764c>

是否有这种关联建模更好的办法？为什么build_parent未定义？

UPDATE：

现在可以与以下控制器代码：

@section = Section.new(params[:section]) 
unless @section.parent_id.empty? 
    @parent = Section.find(@section.parent_id) 
    @section.parent = @parent 
end 

寻找任何建议，它如何能够得到改善，为什么以前没有工作...

Answer 1

你正在寻找一个self join。

class Section < ActiveRecord::Base 
    has_many :subsections, :class_name => "Section" 
    belongs_to :parent_section, :class_name => "Section", 
    :foreign_key => "parent_id" 
end