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Ruby stubbing and mocking with rr

Double Ruby, or rr for short, is a “test double framework” for Ruby. According to the rr README, “A Test Double is a generalization of something that replaces a real object to make it easier to test another object. It’s like a stunt double for tests. The following are test doubles: Mocks, Stubs, Fakes, Spies, Proxies”. It is in the same niche as frameworks like mocha, flexmock, and the mocking support built into RSpec.

So why bother with another framework when the others have been well established and work well? I have two main reasons:

  • Terser syntax that’s more rubyish (you call the methods you want to mock, with the parameters you want)
  • More advanced functionality (test spies are my particular favorite)

I’m going go over the functionality I’ve been using, with a few asides when it’s much different than mocha and RSpec.


The rr gem is, well, rr:

sudo gem install rr

Then you must update your helper to use rr. In test/unit, you’d update test/test_helper.rb to include:

require 'rr'

class Test::Unit::TestCase
  include RR::Adapters::TestUnit

For RSpec, you’d want to update spec/spec_helper.rb to include:

require 'rr'

Spec::Runner.configure do |config|
  config.mock_with RR::Adapters::Rspec


If after installing RR, you get errors like “stack level too deep (SystemStackError)” coming from inside RR, there’s a good chance your helper is being loaded multiple times from different paths.

For example, test/test_foo.rb does require File.join(File.dirname(__FILE__), 'test_helper'), and test/foo/test_bar.rb does require File.join(File.dirname(__FILE__), '..', 'test_helper'). This means that test/test_helper and test/bar/../test_helper are both being required. They have have different paths even though they refer to same file because the ruby interpreter isn’t smart enough to normalize it.

Doing a proper fix for this multiple-require problem in your project is beyond the scope of this article, but here’s a quick fix:

class Test::Unit::TestCase
  include RR::Adapters::TestUnit unless include?(RR::Adapters::TestUnit)


Stubbing lets you replace the implementation of an object, no strings attached.

This will stub away any all calls to trogdor.burninate, regardless of the values and number of arguments.


You’ll often want to make a stub return a specific value. You can specify this with a block or returns. I’ve generally found the former to be more readable.

stub(countryside).burninated? { true }

If you need to stub a method to raise an exception, you can use the same mechanic for returning a value. Just use a block to define a return value, raise something instead of returning.

stub(trogdor).come_in_the_day { raise "the Trogdor comes in the NIIIIIIIIIIIIGHT!!!!!" }

As I mentioned, all these examples will match any invocation of stubbed method. You can be pickier though, and give the arguments you expected it to be called with. You can just pass the values you want to the stub. You can also fallback to using with:



Mocking is similar to stubbing, except it adds an expectation. If you mock something, and it isn’t called, then your test will fail.

You can mostly take the examples above, and replace stub with mock, and away you go.

mock(countryside).burninated? { true }

There is one difference that I’ve seen though. If you mock a method without any arguments… it will only match being called without any arguments. This is different than how mocha/rspec work.

mock(inbox).from_a_female? { false }
inbox.from_a_female?(email) # ERROR! expected no args, but got one arg!

One way around this is to use rr’s wildcard matchers. I’m not going to detail all the possibilities, but I’ll just say we can use anything to match any argument passed to it. Check the README for other matchers.

mock(inbox).from_a_female?(anything) { false }
inbox.from_a_female?(email) # No error, but still not from a female

Don’t allow method calls

rr lets you indicate a method should never be called, and will raise an error if it does. You can use the same style above for specifying argument.


Mock and stub objects

Sometimes you might want to make a pure mock object, without using a specific implementation. rr doesn’t support this as well as mocha or rspec. You can create bare @Object@s, and stub or mock as necessary.

cottage =
stub(cottage).burninated? { true }

(There is a mock! helper for making pure mock objects, but I haven’t had consistently good experiences with it.)

Instances of

Want all your peasants to be burninated in one fell swoop?

stub.instance_of(Peasant).burninated? { true }

You can mock it as well, and specify arguments.

Test spies

Test spies let you stub a method, and then latter assert the values it was called with.



assert_received(trogdor) {|trogdor| trogdor.burninate(countryside) } # test/unit
trogdor.should have_received.burninate(countryside) # rspec
trogdor.should have_received(:burninate).with(countryside) # rspec, same thing

A more detailed example

I have found this extremely useful with rspec and shoulda, where you might stub in a before/setup block, and then have tests that add expectations on those being called. Consider this example using shoulda and mocha:

context "coming in the night" do
  setup do

  before_should "burninate the countryside" do

  before_should "burninate the peasants" do

before_should is used to set an expectation before the setup block actually gets called. This is kind of annoying because you have specify the return values each time. We’re able to express this with RR much more concisely, and without having to rely on before_should:

context "coming in the night" do
  setup do
    stub(trogdor).burninate { true }

  should "burninate the countryside" do
    assert_received(trogdor) {|trogdor| trogdor.burninate(countryside) } 

  should "burninate the peasants" do
    assert_received(trogdor) {|trogdor| trogdor.burninate(peasants) } 


There’s nothing special about using rr for Rails. If you’re using rspec, though, you may want to checkout rspec-rr which provides an rr-ified mock_model and stub_model. It does not seem to support :null_object => true though.


Method missing

rr doesn’t always play nice with stubbing/mocking methods that are provided by method_missing. I’ve had particularly bad times with ActionMailer::Base subclasses.

The issue is that rr works by doing alias_method down in its inner bowls, and you can’t alias a method that doesn’t exist. rr works around this by trying to send to the non-existent method, hoping that the act of sending will define it before trying to send again. It does so without any arguments, so if you are stubbing a method that has any arguments, you are going to see some funky @ArgumentError@s.

One work around for this is to stub out method missing itself, and pass it the name of the method you care about. I found the slightly alternative syntax works best:

stub(PeasantMailer, :deliver_burnination).with(peasant)

You should be able to use test spies on this too:

PeasantMailer.should have_received.method_missing.with(:deliver_burnination, peasant)

Test spies for assignment

There’s a possibility you would want to use test spies for while testing assignments:

stub(trogdor).species = 'dragon'

If you were using test/unit, you’d verify it like:

assert_received(trogdor) {|trogdor| trogdor.species = 'dragon' }

There’s a good chance you’ll get a message to the effect call isn’t defined on the String ‘dragon’. This is caused because inside the block, rr is doing some trickery with method_missing to record what methods you call on trogdor. I’m told that when you do assignment, the ruby interpreter will always return the assigned value, so rr gets confused when it gets a string, rather than it’s own objects to track method calls.

The ugly fix for this is to use it’s method_missing more directly.

assert_received(trogdor) {|trogdor| trogdor.method_missing(:species=, 'dragon') }

Anything else?

I came In reality, I’m only beginning to scratch the surface of rr. The terse syntax and test spies are the things that got me hook, but there are other features that look promising, like proxying and stubbing method chains.

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