custom_validators.rb

# app/validators/uuid_validator.rb
class UUIDValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    UUIDTools::UUID.parse_hexdigest(value)
  rescue ArgumentError
    UUIDTools::UUID.parse(value)
  rescue ArgumentError
    record.errors[attribute] << (options[:message] || "is not a valid UUID")
  end
end


# spec/validators/uuid_validator_spec.rb

require 'spec_helper'

describe UUIDValidator do

  let(:validator) { UUIDValidator.new({:attributes => {}}) }

  let(:model) { mock("model") }

  before(:each) do
    model.stub("errors").and_return([])
    model.errors.stub("[]").and_return({})
    model.errors[].stub("<<")
  end

  it "should not add errors on valid uuid" do
    model.should_not_receive("errors")
    validator.validate_each(model, :uuid, "01234567-8901-2345-6789-012345678912")
  end

  it "should add errors on an invalid uuid" do
    model.errors[].should_receive('<<').with("is not a valid UUID")
    validator.validate_each(model, :uuid, "notvalid")
  end
end