TL;DR, use explicit module namespacing or avoid complex namespacing as often as possible when inheriting classes.
Class and Module constants depend on the order of autoload-paths lookup (a Ruby on Rails feature), more on that below. Because of this, class discoverability can vary depending on the name and location of an inheriting class. When multiple classes share the same name, as is the case in our Rails app, this subtlety can impact implicit class inheritance.
Ruby on Rails has two main algorithms for constant lookup in its
autoload_paths (by default all subdirectories of
app/ in the application and engines present at boot time, and
The values of
autoload_paths can be inspected with the following command:
$ bin/rails r 'puts ActiveSupport::Dependencies.autoload_paths' .../app/assets .../app/controllers .../app/helpers .../app/mailers .../app/models .../app/resources # This is loaded before models/ .../app/controllers/concerns .../app/models/concerns .../test/mailers/previews
In my attempt to clean up some of our code, I unwittingly created a difficult bug to spot. Here is some example code to see the issue in action:
# app/models/campaign.rb # This class is completely irrelevant to our code but has the same class name as the other campaign class. class Campaign; end # app/resources/homepage/apple.rb module Resources module Homepage class Apple < Campaign campaign_method! # This fails because apple.rb is loaded before resources/homepage/campaign.rb end end end # app/resources/homepage/campaign.rb module Resources module Homepage class Campaign def self.campaign_method! "I should be callable" end end end end # app/resources/homepage/yolo.rb module Resources module Homepage class Yolo < Resources::Homepage::Campaign campaign_method! # This works because the parent class is unambiguous end end end # app/resources/homepage/zolo.rb module Resources module Homepage class Zolo < Campaign campaign_method! # This works by luck because Zolo is loaded alphabetically after resources/homepage/campaign.rb end end end
I was able to debug this issue using
pry(main)> Resources::Homepage::Yolo # returns a constant => Resources::Homepage::Yolo pry(main)> Resources::Homepage::Apple # returns an error message NameError: undefined local variable or method `campaign_method!' for Resources::Homepage::Apple:Class`
class Apple < Campaign to
class Apple < Resources::Homepage::Campaign and reloading the console, the code loads correctly.
Explicitly loading the required file also works, which is what Rails AutoLoader (usually) does correctly for you. The Rails Guides discourages using
require_relative for autoloaded files. Instead, use
# resources/homepage/apple.rb require_dependency 'resources/homepage/campaign' module Resources module Homepage class Apple < Campaign campaign_method! # this works and loads campaign's class method end end end
Hopefully, Ruby on Rails autoloading is clearer now. You can always read more and improve Ruby on Rails documentation here. If you have any questions or found errors, please comment here or contact me on twitter!
In Rails 5, autoloading is disabled by default in production.