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

# lib/my_engine/engine.rb
module MyEngine
  class Engine < Rails::Engine
    config.after_initialize do
      require 'my_engine/admin/articles'
    end
  end
end