Message chains rspec vs minitest

Created: 2020-05-30, Last Updated: 2023-02-04

In the Ruby world there are two primary testing frameworks, rspec and minitest. Rails prefers minitest, but it's not hard at all to swap it out with rspec. Full disclosure, I prefer rspec as of right now, and most of the applications I work with use rspec.

A few weeks ago I ran into an issue. I had an app that my team owned that did use minitest, and it was completely foreign. To complicate things it used Minitest::Spec. This threw me off because not all of the rspec things worked. However, I can see why people like it. Personally, I'd prefer using the core minitest way.

Anyway, I ran into a problem. How do you stub a method that's on a chain?

For example, car.get_in.buckle_up_buckaroo!

First, a note about doubles, stubs, dummies, mocks, spies, fakes, monkey patching, centurions, etc!

I was kidding about centurions. Those don't exist...yet. However, the others do, and they caused me grief. What's the difference? Why would I use one over the other?

Now I may update this as I learn more, but there aren't that many differences. Different frameworks have come up with different names for what they needed. That said, defining each one deserves it's own post. However, for the purposes of this post I'm going to use the definitions in the post by Ilija Eftimov.

The problem

I have something I want to test, but it depends on something else. Normally, I'd just stub the needed thing and move on, but what if that thing is owned by another thing. To make matters worse the application is spaghetti code heaven, and I can't trace where things are to do a proper stub? For me I'd rather stub the message chain.

Here is a contrived example.

class GrumpyGameFairy < StandardError
  def initialize(msg="Outlook is cloudy, try again later")
    super
  end
end

class GameFairy
  def proclamation
    if mystic_visions == 'The game is in a stalemate!'
      false
    elsif mystic_visions == 'The game is won!'
      true
    else
      raise GrumpyGameFairy
    end
  end

  private

  def mystic_visions
    "Leave me alone!"
  end
end

class GameFairyGateway
  def self.get_fairy
    game_fairy = GameFairy.new()
  end
end

class Game
  def done?
    return GameFairyGateway.get_fairy.proclamation
  end
end

Here we have a Game, GameFairyGateway, and a GameFairy. The GameFairy is a mysterious resource that we can't understand. How it works is beyond human comprehension. However, it tells us the state of the game (i.e. is the game done), but we fear asking it questions directly. To help us here we've added a GameFairyGateway to be our messager for the GameFairy's divine messages.

The problem is that we want to test the game, but let's say we can't efficiently stub the supernatural nature of the GameFairy How do we do it?

rspec's receive_message_chain

require_relative '../game'

describe Game do
  subject(:game) { Game.new() }
  describe '#done' do

    context 'the game fairy says there is a tie' do
      it 'returns false' do
        allow(GameFairyGateway).to receive_message_chain(:get_fairy, :proclamation) { false }
        expect(game.done?).to be false 
      end
    end

    context 'the game fairy says the player_1 is the winner' do
      it 'returns true' do
        allow(GameFairyGateway).to receive_message_chain(:get_fairy, :proclamation) { true }
        expect(game.done?).to be true 
      end
    end
  end
end

Here we are using receive_message_chain to allow us to base the stub on GameFairyGateway even though we are stubbing the proclamation method on GameFairy. Note that this is really only for legacy code that you're looking to refactor later but for now you can't. Please refer to rspec's documentation for more information.

minitest's solution.

require "minitest/autorun"
require_relative '../game'

class TestGame < Minitest::Test
  def setup
    @game = Game.new
    @game_fairy = Object.new
  end

  def test_that_game_is_not_done

    def @game_fairy.proclamation
      false
    end

    GameFairyGateway.stub :get_fairy, @game_fairy do
      assert_equal false, @game.done? 
    end
  end

  def test_that_game_is_done

    def @game_fairy.proclamation
      true 
    end

    GameFairyGateway.stub :get_fairy, @game_fairy do
      assert_equal true, @game.done? 
    end
  end
end

This case is slightly different as we don't have an equivalent to the receive_chained_message method. In order to do something similar we need to create two stubs. The first uses the @game_fairy dummy and is where we stub out the proclamation method. The second is where we stub out the get_fairy method on GameFairyGateway. Note that these are different ways to do the same thing.

You'd be correct in pointing out that we are stubbing out the thing that we "weren't able to understand". Unfortunately, we don't have another option, so we will have to use a dummy to represent the great power of the GameFairy.

Running the tests

Running the code here is pretty easy. The code can be found on my github profile. Run git clone to get the repo and ensure you have ruby, minitest, and rspec installed.

From there run the rspec suite with: bash rspec rspec/game_spec.rb The minitest suite with: bash ruby minitest/test_game.rb Conclusion

receive_message_chain is not a method that should be used in new code. It's a code smell that indicates unnecessary complexity. minitest does not have an equivalent to it, but be wary of needing to stub out too many things in your unit tests. More is not always better.