RSpec - mock (or stub) overriden mixin method

Question

I have situaltion like this:

module Something
  def my_method
    return :some_symbol
  end
end

class MyClass
  include Something

  def my_method
    if xxx?
      :other_symbol
    else
      super
    end
  end
end

Now the problem is with testing - I would like to ensure that super method got called from overriden method and stub it so that I can test other parts of method. How can I accomplish that using RSpec mocks?

Answer 1

Ensuring that super gets called sounds a lot like testing the implementation, not the behaviour, and mocking the subject-under-test isn't such a great idea anyway. I would suggest just explicitly specifying the different code paths

describe "#my_method" do
  it "returns :other_symbol when xxx" do
    ...
  end

  it "returns :some_symbol when not xxx" do
    ...
  end
end

If you had a lot of classes that included that module, you could use shared examples to reduce the duplication in your tests.

shared_examples_for "Something#my_method" do
  it "returns :some_symbol" do
    expect(subject.my_method).to eq :some_symbol
  end
end

describe MyClass do
  describe "#my_method" do
    context "when xxx" do
      subject { ... }

      it "returns :other_symbol" do
        expect(subject.my_method).to eq :other_symbol
      end
    end

    context "when not xxx" do
      subject { ... }

      it_behaves_like "Something#my_method"
    end
  end
end

Update: If you really can't predict the behaviour of the mixin, you could switch out what method gets called by super by including another module that defines it.

If you have a class C that includes modules M and N that both define a method f, then in C#f, super will refer to whichever module was included last.

class C
  include M
  include N

  def f
    super # calls N.f because it was included last
  end
end

If you include it in the singleton class of your subject-under-test, then it won't affect any other tests:

describe MyClass do
  describe "#my_method" do
    it "calls super when not xxx" do
      fake_library = Module.new do
        def my_method
          :returned_from_super
        end
      end

      subject.singleton_class.send :include, fake_library

      expect(subject.my_method).to be :returned_from_super
    end
  end
end

Disclaimer: this doesn't actually test that the mixin works, just that super gets called. I still would advise actually testing the behaviour.