undefined方法`[]'为nil：NilClass for Rails模型为new

Question

I have a model Student and a controller Students. In the controller I have a new method

def new
  @student = Student.new
end

I then have a /views/students/new.html.haml that freaks out by using the @student. I get the error

   undefined method `[]' for nil:NilClass

This is the haml for it

%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"

If I replace @student with Student.new I get the same results.

However if I start the server new and just use

= Student.new

I will get this output

#<Student:0x007fdf7a9c0778>

If I go into the rails console I can mess around with Students and use Student.new and Student.all and etc.

Does anyone have any idea what's going on or what I'm doing wrong?

Here is the error log from when I load localhost:3000/students/new

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:in `block in start_thread'

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

Student model

class Student < ActiveRecord::Base
end

I just generated this model not too long ago with rails g model Student and then the controller with rails g controller Students

My migration file looks like this:

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

Running Student.new.email = "foo" gives me

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!

Answer 1

You're breaking things with t.string :class

@student.class needs to return Student for ActiveRecord to do its magic.

Your @student.class is going to return nil . That's a problem.

Answer 2

Why have you added a class attribute to Student ? class is a Ruby keyword: someobject.class returns the class of someobject . You have added an attribute with same name, destroying this logic. Change the name of the attribute class to something else.