Extending Rails Engine controller method without duplicating it

Question

How to extend a controller method from a Rails Engine without having to duplicate the whole thing?

Trying to extend https://github.com/radar/forem/blob/rails4/app/controllers/forem/forums_controller.rb -- app/decorators/controllers/forem/forums_controller_decorator.rb :

Ideal

Forem::ForumsController.class_eval do
  def show
    # A simple `include` here or something?

    # New code goes here...
  end
end

Current

Forem::ForumsController.class_eval do
  def show

    # Repeat ALL the code from:
    # https://github.com/radar/forem/blob/rails4/app/controllers/forem/forums_controller.rb

      authorize! :show, @forum
      register_view

      @topics = if forem_admin_or_moderator?(@forum)
        @forum.topics
      else
        @forum.topics.visible.approved_or_pending_review_for(forem_user)
      end

      @topics = @topics.by_pinned_or_most_recent_post

      # Kaminari allows to configure the method and param used
      @topics = @topics.send(pagination_method, params[pagination_param]).per(Forem.per_page)

      respond_to do |format|
        format.html
        format.atom { render :layout => false }
      end

    # New code goes here...
  end
end

Answer 1

We use this gem for multiple applications and engines to do exactly what you want:

https://github.com/EPI-USE-Labs/activesupport-decorators

Answer 2

I could extend a controller method from a Rails Engine without having to duplicate code using alias_method

module ValueSets
  SetsController.class_eval do
    def new_with_authorize
      new_without_authorize
      authorize @value_set
    end

    alias_method :new_without_authorize, :new
    alias_method :new, :new_with_authorize
  end
end