how to use thrid party plugin in rails3

Question

class Category < ActiveRecord::Base
  acts_as_tree :order=>position
  scope :top, where("parent_id is null")
  scope :sub, where("parent_id is not null")
  before_create:add_uuid
  private
    def add_uuid
      require 'uuid'
      self.uuid=UUID.new.generate
    end

end

i used a uuid plugin into rails

i have test the code UUID.new.generate in rails console enviroment it's working fine. but when i put it to Category Model,it didn't work,i'm newbie on rails,hope someone could help me