Beware of Double Stubs in RSpec

class Burger
  attr_reader :options

  def initialize(options={})
    @options = options
  end

  def apply_ketchup(number=0)
    @ketchup = @options[:ketchup]
    # the number is passed from the tests below to make it easier to
    # monitor execution of this method.
    printf "Ketchup applied %d times\n", number
  end

  def apply_mayo_and_ketchup(number=0)
    @options[:mayo] = true
    apply_ketchup(number)
  end

  def has_ketchup_on_it?
    @ketchup
  end
end

describe Burger do
  describe "#apply_mayo_and_ketchup" do
    context "with ketchup and single stubs" do
      let(:burger) { Burger.new(:ketchup => true) }

      it "1: sets the mayo flag to true, ketchup is nil" do
        # this line stubs-out the apply_ketchup method
        # and @ketchup will remain nil b/c the original
        # method is not executed at all
        expect(burger).to receive(:apply_ketchup)
        burger.apply_mayo_and_ketchup(1)

        expect(burger.options[:mayo]).to eq(true)
        expect(burger.has_ketchup_on_it?).to be(nil)
      end

      it "2: sets the mayo and ketchup flags to true" do
        # this line stubs-out the apply_ketchup method
        # but in the end calls the non-stubbed out version as well
        # so that has_ketchup_on_it? will return true !
        expect(burger).to receive(:apply_ketchup).and_call_original
        burger.apply_mayo_and_ketchup(2)

        expect(burger.options[:mayo]).to eq(true)
        expect(burger.has_ketchup_on_it?).to eq(true)
      end
    end

    context "with ketchup and double stubs" do
      let(:burger) { Burger.new(:ketchup => true) }
      before {
        # this line creates a stub for the apply_ketchup method
        allow(burger).to receive(:apply_ketchup)
      }

      it "3: sets the mayo flag to true, ketchup is nil" do
        # this line creates a second stub for the fake apply_ketchup method
        # @ketchup will remain nil b/c the original method which sets its value
        # isn't actually executed. we may as well comment out this line and
        # this will not affect the test at all
        expect(burger).to receive(:apply_ketchup)
        burger.apply_mayo_and_ketchup(3)

        expect(burger.options[:mayo]).to eq(true)
        expect(burger.has_ketchup_on_it?).to be(nil)
      end

      it "4: sets the mayo and ketchup flags to true" do
        # this line creates a second stub for the fake apply_ketchup method.
        # .and_call_original will traverse up the stubs and call the original
        # method. If we don't want to assert that the method has been called
        # or we don't care about its parameters, but only the end result
        # that system state has been changed then this line is redundant!
        # Don't stub & call the original, just call the original method, right?
        # Commenting out this line will cause a failure due to the first stub
        # in before() above. The first stub will execute and @ketchup will remain
        # nil! To set things straight also comment out the allow() line in
        # before()!
        expect(burger).to receive(:apply_ketchup).and_call_original
        burger.apply_mayo_and_ketchup(4)

        expect(burger.options[:mayo]).to eq(true)
        expect(burger.has_ketchup_on_it?).to eq(true)
      end
    end
  end
end