gistfile1.rb

# generate a validator proc 
def generate_access_token
  validator = ->(x){ !self.class.exists?(token: token) }
  self.token = TokenGenerator.generate!(&validator)
end

token_generator.rb

require 'securerandom'

class TokenGenerator
  def self.generate!(&validator)
    begin
      token = SecureRandom.hex(8)
    end until validator.call(token)
    token
  end
end