Rails模型新建时出现undefined method `[]' for nil:NilClass错误

Question

我有一个名为Student的模型和一个名为Students的控制器，在控制器中我有一个新方法。

def new
  @student = Student.new
end

我有一个/views/students/new.html.haml文件，其中使用了@student变量，导致出现错误。

   undefined method `[]' for nil:NilClass

这是对它的 HAML

%h1 Students#new
%p Find me in app/views/students/new.html.haml

= form_for(@student) do |f|
  = f.text_field :email
  = f.button "Submit"

如果我用Student.new替换@student，我将得到相同的结果。但是，如果我启动一个新服务器并只使用.....

= Student.new

我将获得以下输出。

#<Student:0x007fdf7a9c0778>

如果我进入Rails控制台，就能玩弄学生对象并使用Student.new和Student.all等命令。有谁知道这是怎么回事或者我做错了什么吗？以下是当我加载localhost:3000/students/new时的错误日志。

Started GET "/students/new" for 127.0.0.1 at 2014-01-22 20:53:50 -0600
Processing by StudentsController#new as HTML
Completed 500 Internal Server Error in 1ms

NoMethodError - undefined method `[]' for nil:NilClass:
activerecord (4.0.2) lib/active_record/attribute_methods/read.rb:84:in `read_attribute'
activerecord (4.0.2) lib/active_record/attribute_methods/read.rb:59:in `__temp__36c6163737'
activerecord (4.0.2) lib/active_record/core.rb:171:in `initialize'
activerecord (4.0.2) lib/active_record/inheritance.rb:27:in `new'
app/controllers/students_controller.rb:3:in `new'
actionpack (4.0.2) lib/action_controller/metal/implicit_render.rb:4:in `send_action'
actionpack (4.0.2) lib/abstract_controller/base.rb:189:in `process_action'
actionpack (4.0.2) lib/action_controller/metal/rendering.rb:10:in `process_action'
actionpack (4.0.2) lib/abstract_controller/callbacks.rb:18:in `block in process_action'
activesupport (4.0.2) lib/active_support/callbacks.rb:383:in `_run__289605414020287583__process_action__callbacks'
activesupport (4.0.2) lib/active_support/callbacks.rb:80:in `run_callbacks'
actionpack (4.0.2) lib/abstract_controller/callbacks.rb:17:in `process_action'
actionpack (4.0.2) lib/action_controller/metal/rescue.rb:29:in `process_action'
actionpack (4.0.2) lib/action_controller/metal/instrumentation.rb:31:in `block in process_action'
activesupport (4.0.2) lib/active_support/notifications.rb:159:in `block in instrument'
activesupport (4.0.2) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
activesupport (4.0.2) lib/active_support/notifications.rb:159:in `instrument'
actionpack (4.0.2) lib/action_controller/metal/instrumentation.rb:30:in `process_action'
actionpack (4.0.2) lib/action_controller/metal/params_wrapper.rb:245:in `process_action'
activerecord (4.0.2) lib/active_record/railties/controller_runtime.rb:18:in `process_action'
actionpack (4.0.2) lib/abstract_controller/base.rb:136:in `process'
actionpack (4.0.2) lib/abstract_controller/rendering.rb:44:in `process'
actionpack (4.0.2) lib/action_controller/metal.rb:195:in `dispatch'
actionpack (4.0.2) lib/action_controller/metal/rack_delegation.rb:13:in `dispatch'
actionpack (4.0.2) lib/action_controller/metal.rb:231:in `block in action'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:80:in `dispatch'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:48:in `call'
actionpack (4.0.2) lib/action_dispatch/journey/router.rb:71:in `block in call'
actionpack (4.0.2) lib/action_dispatch/journey/router.rb:59:in `call'
actionpack (4.0.2) lib/action_dispatch/routing/route_set.rb:680:in `call'
rack (1.5.2) lib/rack/etag.rb:23:in `call'
rack (1.5.2) lib/rack/conditionalget.rb:25:in `call'
rack (1.5.2) lib/rack/head.rb:11:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/params_parser.rb:27:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/flash.rb:241:in `call'
rack (1.5.2) lib/rack/session/abstract/id.rb:225:in `context'
rack (1.5.2) lib/rack/session/abstract/id.rb:220:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/cookies.rb:486:in `call'
activerecord (4.0.2) lib/active_record/query_cache.rb:36:in `call'
activerecord (4.0.2) lib/active_record/connection_adapters/abstract/connection_pool.rb:626:in `call'
activerecord (4.0.2) lib/active_record/migration.rb:369:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/callbacks.rb:29:in `block in call'
activesupport (4.0.2) lib/active_support/callbacks.rb:373:in `_run__3101080685994121215__call__callbacks'
activesupport (4.0.2) lib/active_support/callbacks.rb:80:in `run_callbacks'
actionpack (4.0.2) lib/action_dispatch/middleware/callbacks.rb:27:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/reloader.rb:64:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/remote_ip.rb:76:in `call'
better_errors (1.0.1) lib/better_errors/middleware.rb:84:in `protected_app_call'
better_errors (1.0.1) lib/better_errors/middleware.rb:79:in `better_errors_call'
better_errors (1.0.1) lib/better_errors/middleware.rb:56:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/debug_exceptions.rb:17:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/show_exceptions.rb:30:in `call'
railties (4.0.2) lib/rails/rack/logger.rb:38:in `call_app'
railties (4.0.2) lib/rails/rack/logger.rb:20:in `block in call'
activesupport (4.0.2) lib/active_support/tagged_logging.rb:67:in `block in tagged'
activesupport (4.0.2) lib/active_support/tagged_logging.rb:25:in `tagged'
activesupport (4.0.2) lib/active_support/tagged_logging.rb:67:in `tagged'
railties (4.0.2) lib/rails/rack/logger.rb:20:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/request_id.rb:21:in `call'
rack (1.5.2) lib/rack/methodoverride.rb:21:in `call'
rack (1.5.2) lib/rack/runtime.rb:17:in `call'
activesupport (4.0.2) lib/active_support/cache/strategy/local_cache.rb:83:in `call'
rack (1.5.2) lib/rack/lock.rb:17:in `call'
actionpack (4.0.2) lib/action_dispatch/middleware/static.rb:64:in `call'
rack (1.5.2) lib/rack/sendfile.rb:112:in `call'
railties (4.0.2) lib/rails/engine.rb:511:in `call'
railties (4.0.2) lib/rails/application.rb:97:in `call'
rack (1.5.2) lib/rack/lock.rb:17:in `call'
rack (1.5.2) lib/rack/content_length.rb:14:in `call'
rack (1.5.2) lib/rack/handler/webrick.rb:60:in `service'
/Users/daltondick/.rvm/rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/webrick/httpserver.rb:138:in `service'
/Users/daltondick/.rvm/rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/webrick/httpserver.rb:94:in `run'

/Users/daltondick/.rvm/rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/webrick/server.rb:295: 在启动线程时产生了错误

Started POST "/__better_errors/70107129930940/variables" for 127.0.0.1 at 2014-01-22 20:53:51 -0600

学生模型

class Student < ActiveRecord::Base
end

我之前用rails g model Student生成了这个模型，然后用rails g controller Students生成了控制器。

我的迁移文件如下：

class CreateStudents < ActiveRecord::Migration
  def change
    create_table :students do |t|
      t.string          :email
      t.string          :first_name
      t.string          :last_name
      t.string          :major
      t.string          :semester
      t.string          :class
      t.integer         :availability
      t.integer         :status

      t.timestamps
     end
   end
 end

运行 Student.new.email = "foo" 代码将会得到以下结果：

SystemStackError: stack level too deep
from /Users/daltondick/.rvm/rubies/ruby-2.0.0-p353/lib/ruby/2.0.0/irb/workspace.rb:86
Maybe IRB bug!

- Rumel

---

从视图中调用@student。 - MrPizzaFace

请展示你的new.html.haml文件。 - S. A.

这不会将您的 block 传递给 Student.new 而不是 form_for 吗？最好将该方法调用括在括号中。 - Carl Zulauf

我将Student.new用括号括起来，得到了相同的结果。 - Rumel

你可以安全地假设你的“学生”模型有一个“电子邮件”属性并且你已经运行了迁移吗？ - Steve Rowley

Answer 1

使用 t.string :class 可能会导致错误。

@student.class 需要返回 Student ，以便 ActiveRecord 发挥其作用。

@student.class 返回 nil，这是一个问题。

- user229044

---

@Jackson_Sandland 不要改变.class的行为。我认为这非常明显。 - user229044

Answer 2

为什么你给 Student 添加了一个 class 属性？class 是 Ruby 的关键字：someobject.class 返回 someobject 的类。你添加了一个同名的属性，破坏了这个逻辑。请将属性 class 的名称更改为其他名称。

- Ahmad Hussain

---

因为我太蠢了。我正在迁移这个网站，之前的模型没有让我想到它会干扰 Ruby 的 .class。现在已经解决了，感谢你的帮助。 - Rumel

class 是 Ruby 的关键字，而不是 Rails 的关键字。 - user229044

抱歉，我匆忙写错了。 - Ahmad Hussain