Can't dup NilClass… maybe try `unloadable`

If you’re using a model or controller from an engine in Rails 2.3.2 you may encounter some crazy errors from time to time. Errors like: A copy of ApplicationController has been removed from the module tree but is still active! Or sometimes the even weirder one: can't dup NilClass

In one situation where you get A copy of ApplicationController has been removed from the module tree but is still active! it could be because you’re using a plugin or engine that provides controllers and inherits from ApplicationController. Some of the classes inherited or included in your engine controllers may fail to get unloaded and cause trouble after the first request to your system. Add unloadable inside your controller classes in your engine.

The can't dup NilClass error really tricked me though. It seemed to be saying something about duping nil class but not really. It was a lie! Well, almost. See, I had a model in my engine like this:

class Account  :destroy

  attr_accessible :nickname, :email

And I had a module in my app (not in the engine) like this (sort of a reverse-micro-plugin):

module Authentication
  module AccountBuddy
    def self.included(account)
      account.class_eval do
        has_many :characters
      end
    end
  end
end

And when I tried to access account.characters it was all “can’t dup NilClass”. But what it really meant to say was: “I’m returning nil instead of an array containing the characters belonging to this account because I became confused during the loading and unloading of all your crazy ass models. -Love, Rails”

So adding a little unloadable also fixes that.

class Account < ActiveRecord::Base
  unloadable #  :destroy

  attr_accessible :nickname, :email

What a load off.

A better way

Courtesy of Paul (from the comments)

unloadable is now deprecated, I think. As far as I am aware, the “right” way to do this is one of the following:

First, you could put something like this in your plugin’s init.rb file:

# This plugin should be reloaded in development mode.
if RAILS_ENV == 'development'
  ActiveSupport::Dependencies.load_once_paths.reject!{|x| x =~ /^#{Regexp.escape(File.dirname(__FILE__))}/}
end

Second, you could put something like this in your application’s environment.rb file:

config.reload_plugins = true if RAILS_ENV == ‘development’

If the above did not solve your problem:

Courtesy of Evan Owen (from the comments)

Another possible cause of this problem is that ActiveRecord also defines self.included. In order to ensure that ActiveRecord gets the call and can finish loading the model, a call to super needs to be added to the end of self.included like so:

def self.included(account)
  account.class_eval do
    has_many :characters
  end

  super # fixes the "can’t dup nil" issue
end

This did not work in my specific case, but may be the correct solution if you are suffering from a similar but different problem.

Author: Daniel X

Heretic priest of the Machine God. I enjoy crawling around in Jeff Bezo's spaceship, bringing technology to the people, and long walks outside of time and space. View all posts by Daniel X

38 thoughts on “Can't dup NilClass… maybe try `unloadable`”

Polydectes says:

April 29, 2009 at 12:43 am

Thank you for this entry. I’ve been fighting these issues all over the place for about 4 weeks and yours is the first place I’ve found a solution to both issues I’ve been having.

Strange: The “{}Controller has been removed” issue is by design (and has been known and not well documented since 2006 (I found the solution in trac about 3 hours before I found this post).

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Tobias says:

April 30, 2009 at 1:39 am

Thank you so much!

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Dimo says:

May 2, 2009 at 5:05 pm

@Polydectes, @Tobias : You’re welcome, I’m glad I could help!

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derek says:

May 6, 2009 at 2:23 pm

htf did you figure this out? haha.

you just saved my day. i have an app that is built up of all kinds of desert_plugins and shortly after updating to 2.3.2 i started getting that weird “Can’t dup NilClass error.”

my conflict was in a model though. the first time my page would load (after a server start) everything worked fine. the second time the page loaded, i got the NilClass error.

adding ‘unloadable’ to the model in my desert plugin solved it all.

thanks for the tip!

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Paul says:

May 7, 2009 at 3:00 am

unloadable is now deprecated I think. As far as I am aware, the “right” way to do this is one of the following.

First, you could put something like this in your plugin’s init.rb file:
# This plugin should be reloaded in development mode.
if RAILS_ENV == ‘development’
ActiveSupport::Dependencies.load_once_paths.reject!{|x| x =~ /^#{Regexp.escape(File.dirname(__FILE__))}/}
end

Second, you could put something like this in your application’s environment.rb file:
config.reload_plugins = true if RAILS_ENV == ‘development’

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Mans says:

May 21, 2009 at 2:01 pm

Thank you! Thank you! Thank you!

I’ve been beating my head against this obscure error for days, doing work on an engine in rails 2.3.2.

Cut. Paste. Solved.

Now, where was I?

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Daniel X Moore says:

May 23, 2009 at 9:20 am

@paul Thanks for the alternative solutions. I’ll be sure to check them out for my next plugin.

@Mans Happy to have helped!

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Amol Kelkar says:

June 10, 2009 at 2:06 pm

I used Paul’s solution because it evidently affects development environment only. That plus his statement that unloadable might be deprecated.. But anyway thanks to both of you daniel and paul!

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Pingback: A copy of ApplicationController has been removed from the module tree but is still active! And can’t dup NilClass
vrybas says:

June 29, 2009 at 12:24 am

Thank’s man! That’s really helped out. Just awesome )

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Danny says:

July 9, 2009 at 3:02 pm

Thanks man, this post helped me out. I ended up putting:

load_paths.each do |path|
ActiveSupport::Dependencies.load_once_paths.delete(path)
end if config.environment == ‘development’

in my engine’s init.rb file. All good.

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Pingback: Hitting unknown error with "can’t dup NilClass" – Afraha ! The Next Big Thing!
Andy Atkinson says:

August 6, 2009 at 5:15 pm

I’m still trying to figure out a problem that produces the same error message. I’m not completely up to speed on unloadable. We’re not using any plugins or engines here, I’ve reproduced the problem in a sample rails app. The problem happens with Paperclip and multiple file uploads from a flash uploader. The first one works great, but the second POST request produces the error. If anyone could take a look at this that would be great. It is a simple example of file uploads with JS+Flash, currently only working for one upload.

http://github.com/webandy/uploadifytest/tree/master

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Brad Bennett says:

August 13, 2009 at 4:47 pm

I was getting this exact same error, in development mode only. It turned out the problem was in a file in my “lib” directory, which was calling “require” for a couple of models. This *somehow* caused the models to not be loaded properly on subsequent requests (thus it would work the first time), causing the “Can’t dup NilClass” error that has driving me absolutely insane for almost 48 hours.

There a little bit more background on the *why*, plus other, similar scenarios in which this could happen on these two threads:

http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/f54f18f4d4354926?pli=1

http://groups.google.com/group/rubyonrails-core/browse_thread/thread/0675477c3fb23bf2/787b561d166abf53?#787b561d166abf53

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Gary says:

September 4, 2009 at 6:04 am

I haven’t tried yet, but I think Rails 2.3.4 fixes the problem entirely.

http://weblog.rubyonrails.org/2009/9/4/ruby-on-rails-2-3-4

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Phil Ayres says:

September 11, 2009 at 5:19 am

This explains the error that suddenly started in my app. I had added a call to self.dosomething from a scheduled task created by Rufus.Scheduler. This must have loaded my model into the engine or some weird thing, locking up regular ‘interactive’ access to the model.

Thank you!!!!

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Gonzalo says:

September 22, 2009 at 3:44 pm

I’m having a Can’t dup NilClass, but unloadable didn’t do the trick, pitty. Anyway, just a little typo in your post: in the example, you wrote ‘unloabable’. 🙂

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Nico says:

September 25, 2009 at 3:24 am

@Gary: No, I still get a can’t dup NilClass error wit rails 2.3.4

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Kris says:

September 25, 2009 at 5:05 am

It doesn’t seem to be fixed in 2.3.4 and unloadable is deprecated, but thanks to this post I found the solution to add the below to init.rb.

load_paths.each do |path|
ActiveSupport::Dependencies.load_once_paths.delete(path)
end if config.environment == ‘development’

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Fred says:

September 29, 2009 at 6:23 am

@Kris :
which init.rb please? Thanks

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Melinda says:

October 2, 2009 at 1:06 pm

Thanks so much! I was getting can’t dup NilClass when adding a has_many :through on a model that is defined inside an engine. Adding this to my environment.rb solved the problem!

config.reload_plugins = true if RAILS_ENV == ‘development’

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@akorshkov says:

October 21, 2009 at 6:11 am

Thanks, thanks, thanks!
Writing code is a pleasure again 🙂

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Stefan says:

December 4, 2009 at 4:35 am

thank you so much!
@Gary: I’m using 2.3.4 and I had the same problem.

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Felix says:

December 10, 2009 at 4:21 am

@Brad: Ta, my messages were also caused by an unnecessary “require ‘user'” in a file in the lib folder. After removing the “require ‘user'” the problem went away. Another problem that went away when I did this was before_update being called twice in test mode (http://dev.rubyonrails.org/ticket/480).

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crackertime says:

December 19, 2009 at 8:20 pm

Snap this started happening today after I updated my XUbuntu system to the latest. I guess the process fudged my rails files. I am not using engines. Maybe it is because I have a model named User? What gives? I don’t want to go around putting some crazy ass line like unloadable in all my models. That is just stupid in my opinion.

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xain says:

December 23, 2009 at 7:20 am

I still get the problem, and I don’t know how to fix it. The solutions listed above cannot fix it. The only option I can choose now is to downgrade to rails 2.2.2

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Evan Owen says:

March 24, 2010 at 12:31 pm

Actually, the real cause of this problem is that ActiveRecord also defines self.included, and since you’re not calling “super” inside your method, ActiveRecord never gets the call and can’t finish loading your model, causing the “can’t dup nil” errors when you try to call the association.

To fix this, simply do:

def self.included(account)
account.class_eval do
has_many :characters
end
super # fixes the “can’t dup nil” issue
end

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1. Daniel X Moore says:
  
  March 26, 2010 at 9:50 am
  
  Thanks Evan! That’s a better explanation and makes a lot of sense. I’ll try it that way the next time I encounter the issue.
  
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  Reply
Kris says:

March 26, 2010 at 5:44 am

@Fred – in my case the init.rb in my plugin as that is where I was getting the error. It maybe different if you are getting the error for your Rails app itself…

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Nathan Hyde says:

June 1, 2010 at 10:56 am

Just wanted to add yet another method of addressing this issue (when all the others don’t seem to work for you). Credit goes to James Adam for pointing me in the right direction and the comments in this article.

See http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/f54f18f4d4354926?pli=1 for an explanation.

In environment.rb:
Rails::Initializer.run do |config| # etc config.to_prepare do # use load to ensure the added functionality is actually evaluated every time load 'path/to/your/extension.rb' end end

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Abdo says:

August 1, 2010 at 1:22 am

I used Kris fix in the plugin init.rb and it fixed the problem and the “A copy of ApplicationController has been removed from the module tree but is still active!” has gone.
Thanks Kris & Daniel

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Christian says:

November 5, 2010 at 1:54 am

I just ran into the ‘A copy of ApplicationController has been removed from the module tree but is still active!’ problem using Rails 2.3.8 and BrowserCMS 3.1.2.
As soon as I was calling ApplicationController’s methods from my custom controllers, the exception would occur.

Paul’s solution works nicely for me.

Paul:

Second, you could put something like this in your application’s environment.rb file:
config.reload_plugins = true if RAILS_ENV == ‘development’

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Diana says:

February 28, 2011 at 12:47 pm

The “Can’t dup NilClass” error can also happen with Time.parse when you pass in a nil value:

oh_noes = nil
Time.parse(oh_noes)

I assume the parse method dupes the value to make sure it doesn’t modify yours, hence the error.

This is what caused ours. 🙂 Likewise I assume any method that tries to dup without checking for nil first will throw this.

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eddroid says:

May 31, 2011 at 9:01 pm

Solution similar to Nathan Hyde’s above:

Rails 2.3.5

Problem: Dev mode wants to reload before every request. Rails initialization gets messed up when reloading code from plugins and monkeypatches. The plugin or patch isn’t loaded at all or, if it is, things that you expect to be defined at reload time aren’t.

My case: monkeypatching the Tag model in acts_as_taggable_on_steroids to add a has_many association

Solution: Wrap the monkeypatch code in:

require ‘dispatcher’
Dispatcher.to_prepare do
Tag.class_eval do
# patches
end
end

Resources:
http://stackoverflow.com/questions/2397740/how-to-extend-an-unloadable-rails-plugin
http://www.oliyiptong.com/blog/2008/08/22/rails-to_prepare-executing-code-before-each-request/

Just another option in case you don’t want to edit environment.rb.

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Sorrow says:

June 21, 2011 at 3:28 am

The comment by Evan saved my day!

I had the same error when using self.inherited. ActiveRecord::Base also defines this method, not only included – calling super solved the problem with “can’t dup NilClass”. Great post, thanks.

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Rafael says:

August 24, 2011 at 10:49 am

Wao! thanks a lot for this post… you save my day.. I was suffering with this problem

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