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Rails Autoloading Behavior and Module name spacing
# app/models/campaign.rb 
class Campaign; end # This class is completely irrelevent to our code, but has same class name as the other campaign class.

# app/resources/homepage/apple.rb
module Resources
  module Homepage
    class Apple < Campaign
      campaign_method!  # This fails because apple.rb is loaded alphabetically 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 rails console

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`

After changing 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 or require_relative for auto loaded files. Instead use require_dependency

# 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
@fanaugen
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fanaugen commented Aug 31, 2017

Nice writeup. Yeah, Rails’ autoloading of constants is weird magic, and most Rails developers (including myself) don’t really understand how it works and what the consequences of it are, which causes many subtle bugs to be created.

In general, for all kinds of library "magic": everything it does for you it also does to you

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