Controllers not being loaded when mounting a Rails Engine - ruby-on-rails

Question

I am currently creating a Rails engine. So far the engine has a few new routes and a few new controllers. The routes work fine, as I was able to mount them adding the following to config/routes.rb:
mount MyEngine::Engine => '/'
The problem comes with the controllers. When trying to access the actions I have defined, I get:
uninitialized constant MyEngine::ApplicationController
I am a bit surprised, as the documentation suggests that everything inside app/ is autoloaded from the engine. I have even tried to explicitly load the controllers in engine.rb, to no avail:
module MyEngine
class Engine < ::Rails::Engine
isolate_namespace MyEngine
Dir["#{config.root}/app/controllers/**/"].each do |path|
config.eager_load_paths << path
end
end
end
I'm confused. Aren't the contents of app/controllers/ supposed to be autoloaded by the application from the engine?

Answer 1

It looks like your engine's controller is trying to reference an ApplicationController inside your engine, when you might have wanted to inherit from the main application's ApplicationController (correct me if I'm wrong).
I don't know how the controller code in your engine looks like (feel free the share if I am not on the right track), but I assume it looks something like this
module MyEngine
class MyController < ApplicationController
end
end
In this scenario, it requires you to also have defined MyEngine::ApplicationController somewhere, and based on the error message, I assume this controller does not exist.
To solve this, you could either define the engine specific ApplicationController, or you can specify that you want to inherit from the root controller, like this:
module MyEngine
class MyController < ::ApplicationController
end
end
If you later want to be even more flexible in your engine, it is often left to an initializer configuration option to set which controller to inherit from (see example from Devise).
module MyEngine
class MyController < MyEngine.parent_controller.constantize
end
end