轨道中的动态形式，NoMethodError

Question

我是铁路的新手，我正在基于这个railscast构建一些东西，

https://github.com/railscasts/403-dynamic-forms

由于这已经非常过时了，我无法下载它所依赖的一个旧宝石——至少使用RVM——所以我无法真正检查它。

然而，有人将其从rails 3更新为4.2，https://github.com/asang/403-dynamic-forms，但我一直收到undefined method 'fields' for nil:NilClass错误。

产品.rb

class Product < ActiveRecord::Base
  belongs_to :product_type
  serialize :properties, Hash
 validate :validate_properties
 def validate_properties
   product_type.fields.each do |field|
    if field.required? && properties[field.name].blank?
     errors.add field.name, "must not be blank"
    end
  end
 end
end

另一个问题是，当从rails 3更改为rails 4.2时，product_field.rb如何访问:field_type，如果没有控制器，如何访问:name？

Product_field.rb-Rails 3

class ProductField < ActiveRecord::Base
 belongs_to :product_type
 attr_accessible :field_type, :name, :required
end

Product_field.rb-Rails 4.2

class ProductField < ActiveRecord::Base
 belongs_to :product_type
end

产品类型控制器.rb

class ProductTypesController < ApplicationController
 def index
  @product_types = ProductType.all
   respond_to do |format|
     format.html 
     format.json { render json: @product_types }
   end
 end

 def show
   @product_type = ProductType.find(params[:id])
   respond_to do |format|
     format.html 
     format.json { render json: @product_type }
   end
 end

 def new
   @product_type = ProductType.new
   respond_to do |format|
    format.html 
    format.json { render json: @product_type }
   end
  end
 def edit
  @product_type = ProductType.find(params[:id])
 end

 def create
  @product_type = ProductType.new(product_type_params)
  respond_to do |format|
    if @product_type.save
      format.html { redirect_to @product_type, notice: 'Product type was    successfully created.' }
    format.json { render json: @product_type, status: :created, location: @product_type }
    else
    format.html { render action: "new" }
    format.json { render json: @product_type.errors, status: :unprocessable_entity }
    end
   end
  end

def update
  @product_type = ProductType.find(params[:id])
  respond_to do |format|
    if @product_type.update_attributes(product_type_params)
      format.html { redirect_to @product_type, notice: 'Product type was successfully updated.' }
      format.json { head :no_content }
    else
      format.html { render action: "edit" }
      format.json { render json: @product_type.errors, status: :unprocessable_entity }
    end
   end
 end

def destroy
  @product_type = ProductType.find(params[:id])
  @product_type.destroy
  respond_to do |format|
    format.html { redirect_to product_types_url }
    format.json { head :no_content }
  end
 end
 def product_type_params
   params.require(:product_type).permit(
        :name, fields_attributes: [ :field_type, :name, :required ] )
 end
end

产品控制程序.rb

class ProductsController < ApplicationController
      def index
        @products = Product.all
      end
      def show
        @product = Product.find(params[:id])
      end
      def new
        @product = Product.new(product_type_id: params[:product_type_id])
      end
      def edit
        @product = Product.find(params[:id])
      end
      def create
        @product = Product.new(product_params)
        if @product.save
          redirect_to @product, notice: 'Product was successfully created.'
        else
          render action: "new"
        end
      end
      def update
        @product = Product.find(params[:id])
        if @product.update_attributes(product_params)
          redirect_to @product, notice: 'Product was successfully updated.'
        else
          render action: "edit"
        end
      end
      def destroy
        @product = Product.find(params[:id])
        @product.destroy
        redirect_to products_url
      end
      private
      def product_params
        params.require(:product).permit(:name, :price, :product_type_id,
                                    :properties)
      end
    end

_form.html.erb

<%= form_for @product do |f| %>
      <% if @product.errors.any? %>
        <div id="error_explanation">
          <h2><%= pluralize(@product.errors.count, "error") %> prohibited this product from being saved:</h2>
          <ul>
          <% @product.errors.full_messages.each do |msg| %>
            <li><%= msg %></li>
          <% end %>
          </ul>
        </div>
      <% end %>
      <%= f.hidden_field :product_type_id %>
      <div class="field">
        <%= f.label :name %><br />
        <%= f.text_field :name %>
      </div>
      <div class="field">
        <%= f.label :price %><br />
        <%= f.text_field :price %>
      </div>
      <%= f.fields_for :properties, OpenStruct.new(@product.properties) do |builder| %>
        <% @product.product_type.fields.each do |field| %>
          <%= render "products/fields/#{field.field_type}", field: field, f: builder %>
        <% end %>
      <% end %>
      <div class="actions">
        <%= f.submit %>
      </div>
    <% end %>

日志

Completed 500 Internal Server Error in 9ms (ActiveRecord: 0.0ms)
    ActionView::Template::Error (undefined method `fields' for nil:NilClass):
        22:   </div>
        23:
        24:   <%= f.fields_for :properties, OpenStruct.new(@product.properties) do |builder| %>
        25:     <% @product.product_type.fields.each do |field| %>
        26:       <%= render "products/fields/#{field.field_type}", field: field, f: builder %>
        27:     <% end %>
        28:   <% end %>
      app/views/products/_form.html.erb:25:in `block (2 levels) in _app_views_products__form_html_erb__38604707748806799_70122829936940'
      app/views/products/_form.html.erb:24:in `block in _app_views_products__form_html_erb__38604707748806799_70122829936940'
      app/views/products/_form.html.erb:1:in `_app_views_products__form_html_erb__38604707748806799_70122829936940'
      app/views/products/new.html.erb:3:in `_app_views_products_new_html_erb__487839569246893265_70122829438980'

railscast上有人发布了一条关于同样问题的评论，另一人回复说，这与遗漏<%= f.hidden_field :product_type_id %>有关。我错过了什么吗？谢谢你花时间仔细研究这个很可能是愚蠢的错误。

Answer 1

问题在于如何在ProductsController#new:中的此行实例化@product

def new
  @product = Product.new(product_type_id: params[:product_type_id])
end

您引用的是product_type的id，而不是具体引用。由于@product未保存在您的new操作中，因此@product.product_type从未从数据库加载，并且始终为nil。要修复，请加载product_type并直接从新产品中引用它

def new
  product_type = ProductType.find(params[:product_type_id])
  @product = Product.new(product_type: product_type)
end