Question

Say I have code like this:

class Car
  def test_drive!; end
end

class AssemblyLine
  def produce!
    car = Car.new
    car.test_drive!
  end
end

Now, using RSpec I want to test/spec AssemblyLine without exercising Car as well. I hear we don’t do dependency injection in Ruby, we stub new instead:

describe AssemblyLine
  before do
    Car.stub(:new).and_return(double('Car'))
  end

  describe '#produce'
    it 'test-drives new cars' do
      the_new_instance_of_car.should_receive(:test_drive) # ???
      AssemblyLine.new.produce!
    end
  end
end

The problem, as you can see, is with the_new_instance_of_car. It doesn’t exist yet before produce is called, and after produce returns it’s too late to set any method call expectations on it.

I can think of a workaround involving a callback in the stubbed new method, but that’s rather hideous. There must be a more elegant and idiomatic way to solve this seemingly common problem. Right…?

---

Update: here’s how I solved it.

describe AssemblyLine
  def stub_new_car(&block)
    Car.stub(:new) do
      car = double('Car')
      block.call(car) if block
      car
    end
  end

  before { stub_new_car } # to make other tests use the stub as well

  describe '#produce'
    it 'test-drives new cars' do
      stub_new_car { |car| car.should_receive(:test_drive) }
      AssemblyLine.new.produce!
    end
  end
end

Answer 1

You can set an expectation on the test double:

describe AssemblyLine do
  let(:car) { double('Car') }
  before { Car.stub(:new) { car } }

  describe "#produce" do
    it "test-drives new cars" do
      car.should_receive(:test_drive!)
      AssemblyLine.new.produce!
    end
  end
end

You can also call any_instance on the class (as of RSpec 2.7, I think):

describe AssemblyLine do
  describe "#produce" do
    it "test-drives new cars" do
      Car.any_instance.should_receive(:test_drive!)
      AssemblyLine.new.produce!
    end
  end
end