Rails 6 not auto-loading class properly in development (zeitwerk mode)

Question

My understanding is that Rails is able to auto-load classes properly named within the app folder, by default.

It's not happening for me, so I want to see if you can find an obvious issue with my code.

My folders/files:

app
   blueprints
      concerns
         ingredient_blueprint_concern.rb
      liqueur_blueprint.rb
   controllers
   models
   ...

#app/blueprints/concerns/ingredient_blueprint_concern.rb

module Concerns
  module IngredientBlueprintConcern
    extend ActiveSupport::Concern

    included do
      attributes :name
    end
  end
end

# app/blueprints/liqueur_blueprint.rb

class LiqueurBlueprint < BaseBlueprint
  include ::Concerns::IngredientBlueprintConcern

  # ...
end

rails console:

> Concerns
=> NameError (uninitialized constant Concerns
   Did you mean?  Concurrent):

---

It seems related to the new Rails 6 zeitwerk mode. If I go back the the classic mode (with the code below), it seems to work fine:

# application.rb
config.autoloader = :classic

Answer 1

https://github.com/rails/rails/issues/36054

Ok so turns out that the concerns folders are actually in the rails load paths, so it means they shouldnt be namespaced, the same way a class is app/models isn't in the Models namespace.

The fact that they were working in Rails 5 was some sort of side-effect that I wont get into here, but you can read about in the link above.