Object Oriented relationship using Test Driven approach.

main.rb

# Engine stuff

class Engine;end

class MotorcycleEngine < Engine
  def start
    "Wroom!"
  end
end

class CarEngine < Engine
  def start
    "Wrooooooooooom!"
  end
end

module EnginePowered
  def start_engine
    @engine.start
  end

  def travel
    "#{start_engine} #{super}"
  end
end

# Vehicles

class Vehicle
  def travel
    "Traveling in Warp speed with a #{self.class}."
  end
end

class Bike < Vehicle
end

class Car < Vehicle
  include EnginePowered

  def initialize
    @engine = CarEngine.new
  end
end

class Motorcycle < Vehicle
  include EnginePowered

  def initialize
    @engine = MotorcycleEngine.new
  end
end

test_main.rb

require 'minitest/autorun'
require './main.rb'

describe Vehicle do
  let(:vehicle) { Vehicle.new }

  it 'should be able to travel' do
    vehicle.travel.must_equal 'Traveling in Warp speed with a Vehicle.'
  end
end

describe Bike do
  let(:bike) { Bike.new }

  it 'shouldn\'t have an engine' do
    refute_respond_to(bike, :start_engine)
  end

  it 'shouldn\'t start an engine before' do
    bike.travel.must_equal 'Traveling in Warp speed with a Bike.'
  end
end

describe Car do
  let(:car) { Car.new }

  it 'should be able start the engine' do
    car.start_engine.must_equal 'Wrooooooooooom!'
  end

  it 'should start the engine before traveling' do
    car.travel.must_equal 'Wrooooooooooom! Traveling in Warp speed with a Car.'
  end
end

describe Motorcycle do
  let(:motorcycle) { Motorcycle.new }

  it 'should be able start the engine' do
    motorcycle.start_engine.must_equal 'Wroom!'
  end

  it 'should start the engine before traveling' do
    motorcycle.travel.must_equal 'Wroom! Traveling in Warp speed with a Motorcycle.'
  end
end