ActiveAdmin with friendly id

Found solution for the problem:

In your app/admin/[ResourceName.rb] add:

  # app/admin/group.rb

  # find record with slug(friendly_id)
  controller do
    def find_resource
      begin
        scoped_collection.where(slug: params[:id]).first!
      rescue ActiveRecord::RecordNotFound
        scoped_collection.find(params[:id])
      end
    end
  end

This solved my problem.

A better approach to @AndreyDeineko's is to override ActiveAdmin::ResourceController's find_resource method in config/initialisers/active_admin.rb and leverage the methods provided by FriendlyId (5.x at this point):

In config/initialisers/active_admin.rb:

ActiveAdmin.setup do |config|
  # == Friendly Id addon
  ActiveAdmin::ResourceController.class_eval do
    def find_resource
      if resource_class.is_a?(FriendlyId)
        scoped_collection.friendly.find(params[:id])
      else
        scoped_collection.find(params[:id])
      end
    end
  end
  # initial config
end

This looks much cleaner to me, than putting it in the application controller, as it is related to the configuration of Active Admin.

There are cases, when application has quit a few resources, hence in order to keep it DRY there is a nice solution requiring few lines of code for whole application - simply override activeadmin's resource controller.

Create config/initializers/active_admin_monkey_patching.rb file with the following content:

ActiveAdmin::ResourceController.class_eval do
  def find_resource
    finder = resource_class.is_a?(FriendlyId) ? :slug : :id
    scoped_collection.find_by(finder => params[:id])
  end
end

Do not forget to restart the server.