How to Use Activesupport::Configurable with Rails Engine

Rails Gem with model-relationships for Rails Engines

Ok,
so I've found a solution for that.

You can just monkey-patch your Engine with Decorators (you need to put it into config/initializers/initializer_name.rb)

see: Extending a ruby gem in Rails

Don't know if this is a good thing, but it works like a charm...

The downside is that you have to restart the server every single time you make a change to the monkey-patching...

EDIT: It seems that this monkey-patch will be garbage collected after a few requests.

EDIT 2:
This post helped me out
How to monkey-patch code that gets auto-loaded in Rails?
you need to add thin sin your Intializer to force rails to reload your patch for every request

Rails.configuration.to_prepare do

Rails' with Activesupport 3.2.0

Looks like you are running on Ruby 1.8.7, but the code you have written is only valid in Ruby 1.9.

If possible, I would recommend upgrading up to 1.9.3; that should fix the issue.

If you cannot use Ruby 1.9.3, just change the line from

key: '_blog_session'

to

:key=>'_blog_session'

How to mount Rails Engines automatically/dynamically?

module MyEngine
  class Engine < ::Rails::Engine
    isolate_namespace MyEngine
    config.my_engine = ActiveSupport::OrderedOptions.new
    initializer 'my_engine.configuration' do |app|
      if app.config.my_engine[:mounted_path]
        app.routes.append do
          mount MyEngine::Engine => app.config.my_engine[:mounted_path]
        end
      end
    end
  end
end

And then in the main application in config/application.rb, you can set

config.my_engine.mounted_path = "/some_path_here"

Decorate a Rails engine's helper to include a concern from the main app

So I finally found a way to do this in a "clean" way, as mentioned in the following comment : https://groups.google.com/g/rubyonrails-core/c/PaABJDXnxyo/m/k-QUJEi9a9wJ

The idea is to add an empty placeholder helper in the engine, for example :

module MyEngine
  module ExtendableHelper
  end
end

And then override it in the main app, by adding methods or including concerns, other gems' helpers, etc...

module MyEngine
  module ExtendableHelper
    extend OtherGemFromMainApp::UsefulHelper
    include MainAppConcern
    
    def other_useful_method
      ...
    end
  end
end

Thus the helper used by the engine is the one provided by the app, and the UsefulHelper methods can be called in the engine views.

Rails engines extending functionality

Just if anyone else runs into same issue some time in the future, this is the code I wrote that fixed my problem:

module ActiveSupport::Dependencies
  alias_method :require_or_load_without_multiple, :require_or_load
  def require_or_load(file_name, const_path = nil)
    if file_name.starts_with?(RAILS_ROOT + '/app')
      relative_name = file_name.gsub(RAILS_ROOT, '')
      @engine_paths ||= Rails::Initializer.new(Rails.configuration).plugin_loader.engines.collect {|plugin| plugin.directory }
      @engine_paths.each do |path|
        engine_file = File.join(path, relative_name)
        require_or_load_without_multiple(engine_file, const_path) if File.file?(engine_file)
      end
    end
    require_or_load_without_multiple(file_name, const_path)
  end
end

This will automatically require files from engine before requiring from application if file path starts with 'app'.

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