Rails4/ActiveAdmin/Heroku precompilation workaround

application.rb

# config/application.rb

# ...

module MyApp
  class Application < Rails::Application
    
    # ...
    
    config.before_initialize do
      Dir.glob(Rails.root.join('lib/**/*.rb')) do |c|
        Rails.configuration.cache_classes ? require(c) : load(c)
      end
    end
    
    # ...
  end
end

resource.rb

# lib/extensions/active_admin/resource.rb

module Extensions
  module ActiveAdmin
    module Resource
      extend ActiveSupport::Concern

      included do
        def default_options
          { sort_order: 'id_desc' }
        end
      end
    end
  end
end

::ActiveAdmin::Resource.send(:include, Extensions::ActiveAdmin::Resource)