Set a random token after model initialization http://shorts.jeffkreeftmeijer.com/2014/set-a-random-token-after-model-initialization/

token.rb

module Concerns::Token
  def self.included(base)
    base.after_initialize(:set_token)
  end

  def set_token
    self.token = SecureRandom.hex if new_record?
  end
end

token_test.rb

require_relative '../../test_helper'

describe Concerns::Token do
  [Session, Discussion, Comment, Notification].each do |model|
    describe "for a #{model}" do
      it "automatically generates a token" do
        model.new.token.length.should == 32
      end
    end
  end
end