RubyChinaはどのようにソフト削除を実現しますか?

# coding: utf-8
#    
module Mongoid
  module SoftDelete
    extend ActiveSupport::Concern

    included do
      field :deleted_at, :type => DateTime

      default_scope where(:deleted_at => nil)
      alias_method :destroy!, :destroy
    end 

    def destroy
      if persisted?
        self.update_attribute(:deleted_at,Time.now.utc)
      end 

      @destroyed = true
      freeze
    end 
  end 
end

  include Mongoid::SoftDelete