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Stubbing Is Not Enough

There has been a good amount of buzz within the Rails community lately about "Isolation Testing", "Mock Objects", and "Fast Tests". I think that this is a wonderful trend. Particularly becuase of Corey Haines' emphasis that practicing Test-Driven development means that you need to listen to your tests in such a way that pain causes you to change your design rather than your tests. It has been a long time coming that Rails developers would begin to listen to the pain coming from their tests, start to use test doubles, and start breaking dependencies within their design.This is wonderful, and I applaud the effort.

Yet at the same time, there are a number of ways in which the community can still press listening to their tests further. For example, although "mock objects" have become almost a buzz word recently within the community, the actual objects that are being created by and large are stubs, not mocks. This means that although we're isolating our tests so that they run faster, we're not completely dealing with the underlying coupling that is pulling in all these dependencies, and making our tests slow.

This post is my attempt to lay out how I think we can both improve the speed of our tests, while at the same time improving the quality of our architectures.

Mocks Are Not Stubs, And Stubs Are Not Mocks

The first clarification that I think is necessary within the community is that there is a difference between "mocks", and "stubs". Although they both replace a real object within a test, they do different things. Gerard Meszaros' definition within his book xUnit Test Patterns is helpful here:

"A Test Stub is an object that replaces a real component on which the SUT [system under test] depends so that the test can control the indirect inputs of the SUT. It allows the test to force the SUT down paths it might not otherwise exercise...

A Mock Object is an object that replaces a real component on which the SUT depends so that the test can verify its indirect outputs."

The key difference is in the way you use the fake object you create.

You would use a stub when a method depends on another object for data in the form of a query. Instead of having to setup a real collaborating object (which might include hitting the database, etc), you replace that object with another object that only returns the data that you need, so that you can avoid having to do the setup. By doing this, you isolate the object under test, because the only dependency that the test needs to run is the code for the object itself.

This is what Corey Haines essentially does in his "Fast Rails Tests" talk. First instead of having a method directly inside of his ActiveRecord object, he moves it into a module, that looks like the following:

class ShoppingCart < ActiveRecord::Base
  include ShoppingCartExtensions::CalculatesTotalPrice

  has_many :shopping_cart_products, dependent: :destroy
  has_many :products, :through => :shopping_cart_products

module ShoppingCartExtensions
  module CalculatesTotalPrice
    def total_price,&:+)

Then in order to test the object in isolation he includes that mixin in a dummy object inside of his tests like this:

class DummyCartWithProducts
  include ShoppingCartExtensions::CalculatesTotalPrice

describe "Calculating Total Price" do
  it "returns the total price of the products" do
    cart =
    products = [stub(price: 5), stub(price:10)]
    cart.stub(:products) { products }
    cart.total_price.should == 15

Essentially what he's done is treated the ActiveRecord part of the shopping cart as a collaborator, and created a stub method for its products. This way he doesn't have to add products into the database and then read them out just in order to be able to test that the cart can properly calculate its total price. And as you might have already guessed, the benefit that comes from the stubs is speed.

Why Stubs Are Not Enough

The primary problem that people generally point out in relation to this form of stubbing however is that it leaves you with "brittle tests" which will give you either false positives, or false negatives. So for example (and I know this is a bit contrived, but the total price method is fairly simple) imagine that I changed price to be a money object that returned the currency type, and the value. What would happen to the calculating total price method? Well, since the test for the total price method doesn't use a real product object the test is going to stay green even though when it is used in production it is going to explode. These sorts of examples can also go the other way as well- although I can't think of a way to do this for the shopping cart example - where the tests break in a refactoring, even though the actual behavior of the system hasn't changed.

One might initially think that this whole "fast tests" thing isn't worth it if it means that our tests are going to become brittle. However the right response is to go back to how Corey starts his presentation, by noticing the difference between Test-Driven Development, and Test-First Development. Practicing Test-Driven development means that you need to listen to your tests in such a way that pain causes you to change your design rather than your tests.

So what is wrong with the design of a system that requires stubs in order to test its objects in isolation? That problem is almost always that the system has poor encapsulation. Why? When you need to stub an object, it means that your going to be querying some dependency for information in order to test the behavior of the object. This means that the behavior of whatever object your testing is necessarily dependent upon the behavior of whatever object your stubbing.Now in some cases, maybe this isn't that bad, but in others this coupling is going to cause a ripple of changes throughout your system whenever you have to change the object that needs to be stubbed. This is a flaw in the design, not the tests.

How Mocks Solve This Problem

The solution to the design problem is to encapsulate our objects so that their behavior can only be affected through their public API. One of the primary ways that we can accomplish this is to follow the "Tell, Don't Ask" principle. That is, instead of asking an object for its properties and then making decisions based on that information, we want our objects to communicate with each other by telling other objects to do something. Any time something needs to happen that has to operate on the data outside of the wall of our object, we delegate that to the object that is responsible for that data to do it for us.

Another way to think about this is that any objects collaborators should be playing a different role than the object itself, and each object should have only a single responsibility. If in the course of developing our objects we run into another responsibility/role we want to delegate that task to the object that is playing that role. If you want more information about how this solves the problem, you should read my articles, "Why you should care about encapsulation", and "Why you should care about information hiding".

This then brings us back to using mock objects instead of stubs inside of our tests. Remember that Meszaros defined a mock object as an object that "replaces a real component on which the SUT depends so that the test can verify its indirect outputs". What he is saying is that a mock object stands in for one or more of the roles of an object's collaborators in order for us to verify the commands sent to that object. That is, since some actions do not necessarily modify the state of the isolated object under test, the way that we want to do our assertion is if it sends the right command to its collaborator.

So for example, the following ticket machine interface is an example of a well encapsulated code:

class TicketMachineInterface
  def initialize(ticket_reserver)
    @ticket_reserver = ticket_reserver
    @current_display = ""

  def number_pressed(number)
    @current_display += number.to_s

  def delete_pressed

  def submit_request

If I wanted to test this object in isolation, how would I do that? Well, instead of passing in a real object that would play the role of the ticket reserver, I could pass in a mock object that would allow me to assert on the messages that was sent to it. So for example, my test would look something like the following:

describe TicketMachineInterface do
  it "reserves the number of tickets inputted when the user submits a request" do
    ticket_reserver = double('ticket_reserver')

    machine =

The assertion here is found in the code "ticket_reserver.should_receive(:reserve).with(55)". Instead of asserting on the state of the object, I assert on its behavior. This then allows my test to respect the encapsulation of my object, and isolate it from its dependencies (which means it runs fast), while not making my tests brittle.

The only key becomes mocking roles, instead of objects so that we can let the object decide how to accomplish what it is told to do. And when we follow this "Tell, Don't Ask" style of design it produces flexable code because it then becomes easy to swap or compose objects that play the same role in order to change the behavior of the system.


As I said at the outset, I'm quite happy that there seems to be an increasing interest in Test-Driven Development, and feeling the pain within our tests. I hope this post has given you something to think about, and some ways to think about improving the speed of your test suite without creating brittle tests. I'd love any feedback in the comments.

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