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 < ActiveRecord::Base
  include Authentication::ByCookieToken
  include Authentication::AccountBuddy
 
  has_many :logins, :dependent => :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 # <= That's the ticket!
  include Authentication::ByCookieToken
  include Authentication::AccountBuddy # reverse-micro-plugin 
 
  has_many :logins, :dependent => :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.

Adding a Non-null Column with no Default Value in a Rails Migration

This is something that I’ve often needed to do: add a new column to the DB that has a non-null constraint, but also doesn’t have a default value. There are a some options:

  • Forget the DB constraint and use `validates_presence_of` in the model
  • Add a default value for the new column with non-null and then remove the default
  • Add the column without a default value, then alter it to be non-null

The first method of simply using `validates_presence_of` won’t cut it for me because that doesn’t actually make guarantees on the data stored in the DB from interfaces outside the of application.

Adding the new non-null column with a default value and then altering it to remove the default would probably be the best choice if you just need a standard value for all your historic data.

The third method is how I did it, and I like it best for any data that can be computed to a reasonable value to start out.Here’s the source for my migration:

class AddLoginMetricsToAccounts &lt; ActiveRecord::Migration
  def self.up
    add_column :accounts, :last_login, :datetime
    add_column :accounts, :total_logins, :integer, :null => false, :default => 1
 
    Account.reset_column_information
 
    Account.all.each do |account|
      account.last_login = account.created_at
      account.save!
    end
 
    change_column :accounts, :last_login, :datetime, :null => false
  end
 
  def self.down
    remove_column :accounts, :total_logins
    remove_column :accounts, :last_login
  end
end

I’m adding a last_login column that I want to be non-null, but because it has no default value most DBs won’t allow the new column to be added (it violates data integrity). So the thing is to add the column without a a non-null constraint, populate it with acceptable values, and then to change the column to include the constraint.

Hope this comes in handy!

Pixie – A JavaScript Pixel Editor

As part of this amazing new game that we are working on we have an online pixel editor component. Right now it is a rough proof of concept. The primary goal is to create a simple online editor that provides the right blend of tools to easily create 32×32 pixel images which can then be saved locally or uploaded to the server. You can even load previously uploaded images, sweet!

The inspiration is Pixen, but instead of being Mac-only it will be available online for all platforms. Pixen provides a strong tool set and a great interface, but why be so exclusive about it? I know, right? The browser is the future anyway, and doesn’t crash as often.

Check out Pixie here. The idea is to keep it simple, there are only a few features missing from fullfilling every one of my wildest dreams (undo, selection, semi-transparency). For fun check out the JS code, it was designed with clarity and extensibility in mind. It’s a purely JS/CSS based editor, built using Prototype.js.

Editing images is fun!

Editing images is fun!

To get the images to the server they undergo an amazing (and probably inefficient) journey. Each pixel is read from the “canvas” and put into an ancient (late 90s) JS PNG encoder. That PNG encoder then spits out the image as a png data file which is then base64 encoded and sent to the server. The server base64 decodes the file and saves it to the local file system. Loading images from the server makes it even crazier! Since this ancient PNG encoder doesn’t load PNG data (to my knowledge) the server uses RMagick to read the uploaded image’s pixels and convert them to a JS array of hex color values, then passes that array into a load method back on the client. It’s amazing!

Have fun editing all those little images, and you can even check out the game that goes with it!