Rails 3 Auto-concerner

1_existing_app.rb

class User < ActiveRecord::Base
end

2_blog_post_engine.rb

# Suas engines podem contribuir para a app principal usando Extensions
# AwesomeEngine::Extension.concern! aplica todos os <Model>Concern que ele possui
# ao respectivo Model de maneira "unloadable".
# Isso quer dizer que ao modificar sua Extension, tudo que você precisa é
# fazer reload! no console.
# <3 <3 <3 <3

#app/models/post.rb
class Post < ActiveRecord::Base
  belongs_to :user
end

#lib/blog_post/extensions/post_extension.rb
class PostExtension < AwesomeEngine::Extension
  module UserConcern
    included do
      has_many :posts
    end
  end
end

#lib/blog_post/engine.rb
module BlogPost
  class Engine < Rails::Engine
    include AwesomeEngine::EngineConcern

    config.to_prepare do
      PostExtension.concern!
    end
  end
end

3_comment_engine.rb

#app/models/comment.rb
class Comment < ActiveRecord::Base
  belongs_to :post
end

#lib/blog_comment/extensions/comment_extension.rb
class CommentExtension < AwesomeEngine::Extension
  module PostConcern
    included do
      has_many :comments
    end
  end

  module UserConcern
    included do
      has_many :comments, :through => :posts
    end
  end
end

#lib/blog_comment/engine.rb
module BlogComment
  class Engine < Rails::Engine
    include AwesomeEngine::EngineConcern

    config.to_prepare do
      CommentExtension.concern!
    end
  end
end