Define a resource inside an engine

To define a resource inside of an engine, use the following technique:

# lib/my_engine/admin/articles.rb
if defined?(ActiveAdmin)
  ActiveAdmin.register Article do
    # customize your resource here
  end
end

module MyEngine
  class Engine < ::Rails::Engine
    isolate_namespace MyEngine
    config.after_initialize do
    end

    initializer :my_engine do
      ActiveAdmin.setup do |active_admin_config|
        active_admin_config.load_paths += Dir[File.dirname(__FILE__) + '/my_engine/admin']
      end
    end
  end
end