Generating a grouped collection select with `simple_form`and a model using the `ancestry` gem

_form.html.erb

<%= 
  f.association :category_id, collection: Category.roots, 
                              as: :grouped_select, 
                              group_method: :children, group_label_method: :name, 
                              label_method: :name
%>

category.rb

class Category < ActiveRecord::Base
  attr_accessible :name
  
  has_ancestry orphan_strategy: :adopt  
end