Ancestry in Rails Admin

Category model snippet:

class Category < ActiveRecord::Base attr_accessible :name, :parent_id has_ancestry

Rails admin configuration snippet :

config.model Category do
  field :name
  field :parent_id, :enum do
    enum do
      except = bindings[:object].id
      Category.where("id != ?", except).map { |c| [ c.name, c.id ] }
    end
  end
end