order.rb

ActiveAdmin.register Order do

  # Export the current collection of items with filtering and sorting
  
  # List the current collection ids
  collection_action :export_txt do
    render :text => collection.map { |order| order.id }.join(', ')
  end

  # Add a button to index page to export current collection as txt
  action_item(:only => :index) do
    link_to("Export TXT", export_txt_admin_orders_path(params.except("controller", "action")))
  end
end