A concise way to generate random values until a unique one is found.

generate_token.rb

class Device
  include MongoMapper::Document
  
  key :token, :unique => true, :default = lambda { generate_token }
  
  def self.generate_token
    token = ActiveSupport::SecureRandom.hex until token_available?(token)
    token
  end
  
  def self.token_available?(token)
    token && count(:device_token => token) == 0
  end
  
end