Rails - NilClass 的未定义方法“model_name”：新操作中的类

Question

我不明白为什么，但我的新视图开始崩溃。

我有模型票

class Ticket < ActiveRecord::Base
 attr_accessible :body, :department, :email, :name, :reference, :subject, :status
 has_many :replies, :dependent => :destroy

和简单的新动作

def new
  logger.debug "#{@ticket}"
  @ticket = Ticket.new
  respond_to do |format|
    format.html # new.html.erb
    format.json { render json: @ticket }
  end
end

和new.haml.html(抱歉haml格式错误):

 = form_for @ticket do |f|
 - if @ticket.errors.any?
 #error_explanation
  %h2= "#{pluralize(@ticket.errors.count, "error")} prohibited this ticket from being saved:"
  %ul
    - @ticket.errors.full_messages.each do |msg|
      %li= msg

 .field
  = f.label :name
  = f.text_field :name
 .field
 - if current_admin
 = f.label :email
 = f.text_field :email, :value => "#{current_admin.email}", :disabled => :true
 - else
  = f.label :email
 = f.text_field :email
 .field
 = f.label :department
 = f.select :department, options_for_select(["First department","Second department","Third department"])
.field
 = f.label :subject
 = f.text_field :subject
 .field
  = f.label :body
 = f.text_area :body
- if current_admin
 .field
  = f.label :status
    = f.select :status, options_for_select(["Waiting for Staff Response","Waiting for Customer","On Hold", "Cancelled", "Completed"])
  .actions
    = f.submit 'Save'

错误：

ActionView::Template::Error (undefined method `model_name' for NilClass:Class):
1: = form_for @ticket do |f|
2:   - if @ticket.errors.any?
3:     #error_explanation
4:       %h2= "#{pluralize(@ticket.errors.count, "error")} prohibited this ticket from being saved:"
app/views/tickets/_form.html.haml:1:in   
`_app_views_tickets__form_html_haml__659380982_96991070'
 Rendered /home/denmed/.rvm/gems/ruby-1.9.2-p320/gems/actionpack-3.2.8/lib
/action_dispatch/middleware/templates/rescues/_trace.erb (5.9ms)
 Rendered /home/denmed/.rvm/gems/ruby-1.9.2-p320/gems/actionpack-3.2.8/lib
 /action_dispatch/middleware/templates/rescues/_request_and_response.erb (4.1ms)
Rendered /home/denmed/.rvm/gems/ruby-1.9.2-p320/gems/actionpack-3.2.8/lib
/action_dispatch/middleware/templates/rescues/template_error.erb within rescues/layout (42.2ms)

和routes.rb

 resources :tickets do 
  resources :comments
 end

我可以从控制台创建票证，那么问题出在哪里？有人可以建议错误在哪里吗？

Answer 1

问题是@ticket 不知何故被“消灭”了。那是因为当您查看 form_for 内部时

 def form_for(record, options = {}, &proc)
   # .... 
   object = record.is_a?(Array) ? record.last : record
   object_name = options[:as] || ActiveModel::Naming.param_key(object)
   # ....
end

ActiveModel::Naming.param_key(object) 被调用的地方，我们最终采用以下方法

  def model_name_from_record_or_class(record_or_class)
    (record_or_class.is_a?(Class) ? record_or_class : record_or_class.class).model_name
  end

在那里你可以看到如果你将 nil 传递给这个方法，它会抛出你发布的错误 - 所以检查@ticket 在哪里以及为什么得到nil
为此，您可能需要关注 this guide，它解释了如何调试 Rails 应用程序。

---

您可能还想交换这些行。

def new
    logger.debug "#{@ticket}"
    @ticket = Ticket.new
end

因为它现在的方式你不会得到任何有用的输出。