token_generator.rb

require 'rails_generator/secret_key_generator'

module TokenGenerator
  def generate_token(size = 32, &validity)
    constant = "#{self.class.name}#{id}"
    generator = Rails::SecretKeyGenerator.new(constant)

    begin
      token = generator.generate_secret.first(size)
    end while !validity.call(token) if block_given?

    token
  end
end

user.rb

class User < ActiveRecord::Base
  include TokenGenerator

  before_save :set_token
  
  private
  def set_token
    if self.token.blank?
      write_attribute :token, generate_token(9) { |token| self.class.find_by_token(token).nil? }
    end
    
    true
  end
end