Testing Ruby Mixins with Minitest in isolation

Posted by Ilija Eftimov on November 4, 2015

Mixins in Ruby are a very powerful feature. But knowing how to test them sometimes is not so obvious, especially to beginners. I think that this comes from mixins’ nature - they get mixed into other classes. So, if you think that there is a lot to testing mixins and you easily get overwhelmed by it - take it easy, it’s not that hard.

Let’s see how easy it is to test mixins, with some help from the lovely Minitest.

Mixins

When writing Ruby, we usually write two types of mixins. The first one is a coupled mixin, and the other, well.. uncoupled.

Uncoupled mixins

Uncoupled mixins are the ones whose methods do not depend on the implementation of the class where they will get mixed in.

Quick example:

module Speedable
  def speed
    "This car runs super fast!"
  end
end

class PetrolCar
  include Speedable
  def fuel
    "Petrol"
  end
end

class DieselCar
  include Speedable
  def fuel
    "Diesel"
  end
end

In irb:

>> p = PetrolCar.new
=> #<PetrolCar:0x007fc332cc4be0>
>> p.speed
=> "This car runs super fast!"
>> p.fuel
=> "Petrol"

>> d = DieselCar.new
=> #<DieselCar:0x007fc332cae2f0>
>> d.speed
=> "This car runs super fast!"
>> d.fuel
=> "Diesel"

As you can see, the speed method in the Speedable mixins does not depend on any other methods. In other words, it’s self-contained.

Testing uncoupled mixins

When it comes to testing uncoupled mixins, it’s quite trivial. There are two main strategies that you can use, by extending the singleton class of an object or by using a dummy class.

Let’s see the first one.

class FastCarTest < Minitest::Test 
  def setup
    @test_obj = Object.new
    @test_obj.extend(Speedable)
  end
  
  def test_speed_reported
    assert_equal "This car runs super fast!", @test_obj.speed
  end
end

As you can see, we instantiate an object of the Object class which is just an empty, ordinary object that doesn’t do anything. Then, we extend the object singleton class with the Speedable module which will mix the speed method in. Then, in the test, we assert that the method will return the expected output.

The second strategy is the “dummy class” strategy:

class DummyTestClass
  include Speedable
end

class FastCarTest < Minitest::Test 
  def test_speed_reported
    dummy = DummyTestClass.new
    assert_equal "This car runs super fast!", dummy.speed
  end
end

As you can see, we create just a dummy class, specific only for this test file. Since the FastCar mixin is mixed in, the DummyTestClass will have the speed method as an instance method. Then, in the test, we just create a new object from the dummy class and assert on the dummy.speed method.

Coupled mixins

Coupled mixins are the ones whose methods depend on the implementation of the class where they will be mixed in. Or, the oposite of uncoupled mixins.

Let me show you a quick example:

module Reportable
  def report
    "This car runs on #{fuel}."
  end
end

class PetrolCar
  include Reportable
  def fuel
    "petrol"
  end
end

class DieselCar
  include Reportable
  def fuel
    "diesel"
  end
end

Now, if we try our classes in irb:

>> pcar = PetrolCar.new
=> #<PetrolCar:0x007fda3403bba8>
>> pcar.report
=> "This car runs on petrol."

>> dcar = DieselCar.new
=> #<DieselCar:0x007fda3318a320>
>> dcar.report
=> "This car runs on diesel."

You see, the implementation of the Reportable#report method relies on (or, is coupled to) the implementation of the DieselCar and PetrolCar classes. If we mixed in the Reportable mixin in a class that does not have the fuel method implemented, we would get an error when calling the report method.

Testing coupled mixins

When it comes to coupled mixins, testing can get just a tad bit harder. Again, the same two strategies.

class ReportableTest < Minitest::Test 
  def setup
    @test_obj = Object.new
    @test_obj.extend(Reportable)

    class << @test_obj
      def fuel
        "diesel"
      end
    end
  end
  
  def test_speed_reported
    assert_equal "This car runs on diesel.", @test_obj.report
  end
end

Now, you can see, it gets a bit hairy when we open the singleton class of the @test_obj and we add the fuel method, so our coupled mixin can work. But, otherwise, it’s quite straight forward.

The approach that I prefer is using the dummy class because it’s much more explicit.

class DummyCar
  include Reportable

  def fuel
    "gasoline"
  end
end

class ReportableTest < Minitest::Test 
  def test_fuel_reported
    dummy = DummyCar.new
    assert_equal "This car runs on gasoline.", dummy.report
  end
end

Now, this is cleaner. We create a DummyCar class where we mix the Reportable mixin and we define the fuel method. Then, in the test, we just create a DummyCar object and we assert for the value of the report method. Remember, we are doing this only because we want to test the mixin. If we were to test any of the classes, there would be no point in doing this.

As you can see, it’s quite simple to test these mixins with plain-old Ruby. I guess it’s worth mentioning that these examples are very simple and you might run into more complex mixins that need testing in the future. But, when testing mixins, these strategies will still work. Just don’t get owerwhelmed and take it step-by-step.


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