Don't reopen ActiveRecord in another file

The power of Ruby lies partially in how one can reopen classes to redefine them. Besides namespace clashes, this is usually a good way to extend and refine classes to your own uses. However, last night, I got bitten in the ass trying to refactor a couple classes. In Rails, you're allowed to extend associations by adding a class the association call.

class User < ActiveRecord::Base
  has_many :stories, :through => :entries, :source => :story, 
    :extend => StoryAssociationExtensions
end

where StoryAssociationExtensions is a class module holding methods, like expired() that I can perform on the challenges association, so I can do stuff like

  @user = User.find(:first)
  @user.stories.expired  # gives all expired stories

So when refactoring and cleaning up, I renamed StoryAssociationExtensions to AssociationExtensions and reopened up Story class and put it in there. I just wanted to clean up the namespace, and put the association extensions somewhere that made semantic sense. Naturally, I thought putting association extensions for a class belongs in a class. Well, it doesn't work. And don't do it. Hopefully, I'm saving you some pain.

class Story < ActiveRecord::Base
  module AssociationExtensions
    def expired
      self.select { |c| c.expired? }
    end
  end
end

Well, this works if you've reopened the class within the same model file, story.rb in this case. However, if you reopen the class in another file elsewhere, your model definition won't get loaded properly, which leads to associations and methods you defined not to exist.

So imagine my bewilderment when associations didn't work on only certain ActiveRecord Models. In addition, they worked on the unit tests and script/console, but didn't work when the server was running. All that at 3am in the morning. :(

Good thing for source control, so I could revert (but I have to say, svn isn't as easy to use as it could be).

I ended up creating a directory in model called collection_associations and putting the associations in there under a module CollectionAssociations namespace. Not exactly the best arrangement but it'll do for now.

I'm still not sure why ActiveRecord::Base instances don't like being reopened, but I'm guessing it has something to do with only getting loaded once. If anyone has an explanation, I'd like to read about it.

free warning!