Use in model with `include UUIDPrimaryKey`

model_with_an_uuid.rb

RSpec.shared_examples 'a model with an uuid' do
  subject(:instance) { FactoryGirl.create(described_class.name.downcase) }

  it { should have_readonly_attribute(:id) }

  describe '.find' do
    it 'finds by uuid' do
      expect(
        described_class.find(instance.id)
      ).to eq(instance)
    end
  end

  describe '#uuid' do
    it 'is valid' do
      expect(
        instance.uuid
      ).to match(
        # regexp that matches UUIDs Version 4 (random)
        /\A[a-f0-9]{8}-[a-f0-9]{4}-4[a-f0-9]{3}-[89aAbB][a-f0-9]{3}-[a-f0-9]{12}\z/
      )
    end
  end
end

uuid_identifier.rb

class UUIDIdentifier
  attr_reader :uuid

  def self.from_key(key)
    uuid = "#{key[8..15]}-#{key[4..7]}-#{key[0..3]}-#{key[16..19]}-#{key[20..32]}"
    new(UUIDTools::UUID.parse(uuid))
  end

  def initialize(uuid = nil)
    @uuid = uuid || UUIDTools::UUID.random_create
  end

  def to_s
    uuid.to_s
  end

  def to_key
    s = to_s
    "#{s[14..17]}#{s[9..12]}#{s[0..7]}#{s[19..22]}#{s[24..35]}"
  end
end

uuid_identifier_spec.rb

require 'rails_helper'

RSpec.describe UUIDIdentifier do
  let(:uuid_str) { '6ccd780c-baba-1026-9564-0040f4311e29' }
  let(:uuid_key) { '1026baba6ccd780c95640040f4311e29' }

  describe '.new' do
    before do
      allow(UUIDTools::UUID).to receive(:random_create).and_return(
        UUIDTools::UUID.parse(uuid_str)
      )
    end

    context 'without an UUID' do
      it 'generates UUID and returns UUIDIdentifier' do
        expect(UUIDIdentifier.new).to be_a(UUIDIdentifier)
        expect(UUIDTools::UUID).to have_received(:random_create)
      end
    end

    context 'with a UUID' do
      let(:uuid) { UUIDTools::UUID.parse(uuid_str) }

      it 'uses UUID and returns UUIDIdentifier' do
        expect(UUIDIdentifier.new(uuid)).to be_a(UUIDIdentifier)
        expect(UUIDTools::UUID).to_not have_received(:random_create)
      end
    end
  end

  describe '.parse' do
    it 'returns UUIDIdentifier' do
      identifier = UUIDIdentifier.from_key(uuid_key)
      expect(identifier).to be_a(UUIDIdentifier)
      expect(identifier.to_s).to eq(uuid_str)
    end
  end

  describe '#to_key' do
    let(:uuid) { UUIDTools::UUID.parse(uuid_str) }

    it 'returns UUID key' do
      identifier = UUIDIdentifier.new(uuid)
      expect(identifier.to_key).to eq(uuid_key)
    end
  end

  describe '#to_s' do
    let(:uuid) { UUIDTools::UUID.parse(uuid_str) }

    it 'returns UUID string' do
      identifier = UUIDIdentifier.new(uuid)
      expect(identifier.to_s).to eq(uuid_str)
    end
  end
end

uuid_primary_key.rb

require 'active_support/concern'

module UUIDPrimaryKey
  extend ActiveSupport::Concern

  included do
    attr_readonly :id

    before_create :ensure_id

    def uuid
      UUIDIdentifier.from_key(id).to_s
    end
  end

  private def ensure_id
    self.id ||= UUIDIdentifier.new.to_key
  end

  class_methods do
  end
end