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RR (Double Ruby) is a test double framework that features a rich selection of double techniques and a terse syntax.
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RR (Double Ruby) is a test double framework that features a rich selection of double techniques and a terse syntax.

What is a Test Double?

A Test Double is a generalization of something that replaces a real object to make it easier to test another object. Its like a stunt double for tests. The following are test doubles:

  • Mocks

  • Stubs

  • Fakes

  • Spies

  • Proxies

Currently RR implements mocks, stubs, and proxies. In the future, RR will support spies.

Using RR


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


Spec::Runners.configure do |config|
  config.mock_with :rr
  # or if that doesn't work due to a version incompatibility
  # config.mock_with RR::Adapters::Rspec

Syntax between RR and other double/mock frameworks

Terse Syntax

One of the goals of RR is to make doubles more scannable. This is accomplished by removing words from a double declaration. Here is RR compared to other mock frameworks:

flexmock(User).should_receive(:find).with('42').and_return(jane) # Flexmock
User.should_receive(:find).with('42').and_return(jane) # Rspec
User.expects(:find).with('42').returns {jane} # Mocha
User.should_receive(:find).with('42') {jane} # Rspec using return value blocks
mock(User).find('42') {jane} # RR

No “pure” mock object

RR is an opinionated framework. RR does not create a mock object for you, like other frameworks. Instead, RR utilizes a technique known as “double injection”.

my_object =

Compare this with doing a mock in mocha:

my_mocked_object = mock()

If you wish to use objects for the sole purpose of being a mock, you can do so by creating an empty object.

mock(my_mock_object =

No should_receive or expects method

RR uses method_missing to set your method expectation. This means you do not need to use a method such as should_receive or expects.

mock(my_object).hello # The hello method on my_object is mocked


my_object.expects(:hello) # expects sets the hello method expectation

Rspec mocks:

my_object.should_receive(:hello) # should_receive sets the hello method expectation

with method call is not necessary

Since RR uses method_missing, it also make using the with method unnecessary to set the argument expectations.

mock(my_object).hello('bob', 'jane')


my_object.expects(:hello).with('bob', 'jane')

Rspec mocks:

my_object.should_receive(:hello).with('bob', 'jane')

using a block to set the return value

RR supports using a block to set the return value. RR also has the #returns method. Both of the examples are equivalent.

mock(my_object).hello('bob', 'jane') {'Hello Bob and Jane'}
mock(my_object).hello('bob', 'jane').returns('Hello Bob and Jane')


my_object.expects(:hello).with('bob', 'jane').returns('Hello Bob and Jane')

Rspec mocks:

my_object.should_receive(:hello).with('bob', 'jane').and_return('Hello Bob and Jane')
my_object.should_receive(:hello).with('bob', 'jane') {'Hello Bob and Jane'} #rspec also supports blocks for the return value

Using RR

To create a double on an object, you can use the following methods:

  • mock

  • stub

  • proxy

  • instance_of

These methods are composable. mock and stub can be used by themselves and are mutually exclusive. proxy and instance_of must be chained with mock or stub. You can chain proxy and instance_of together.


mock replaces the method on the object with an expectation and implementation. The expectations are a mock will be called with certain arguments a certain number of times (the default is once). You can also set the return value of the method invocation.


The following example sets an expectation that the view will receive a method call to #render with the arguments {:partial => “user_info”} once. When the method is called “Information” is returned.

view = controller.template
mock(view).render(:partial => "user_info") {"Information"}


stub replaces the method on the object with only an implementation. You can still use arguments to differentiate which stub gets invoked.


The following example makes the User.find method return jane when passed '42' and returns bob when passed '99'. If another id is passed to User.find, an exception is raised.

jane =
bob =
stub(User).find('42') {jane}
stub(User).find('99') {bob}
stub(User).find do |id|
  raise "Unexpected id #{id.inspect} passed to me"


mock.proxy replaces the method on the object with an expectation, implementation, and also invokes the actual method. mock.proxy also intercepts the return value and passes it into the return value block.

The following example makes sets an expectation that view.render({:partial => “right_navigation”}) gets called once and return the actual content of the rendered partial template. A call to view.render({:partial => “user_info”}) will render the user_info partial template and send the content into the block and is represented by the html variable. An assertion is done on the html and “Different html” is returned.

view = controller.template
mock.proxy(view).render(:partial => "right_navigation")
mock.proxy(view).render(:partial => "user_info") do |html|
  html.should include("John Doe")
  "Different html"

You can also use mock.proxy to set expectations on the returned value. In the following example, a call to User.find('5') does the normal ActiveRecord implementation and passes the actual value, represented by the variable bob, into the block. bob is then set with a mock.proxy for projects to return only the first 3 projects. bob is also mocked with valid? to return false.

mock.proxy(User).find('5') do |bob|
  mock.proxy(bob).projects do |projects|
  mock(bob).valid? {false}


Intercept the return value of a method call. The following example verifies render partial will be called and renders the partial.

view = controller.template
stub.proxy(view).render(:partial => "user_info") do |html|
  html.should include("Joe Smith")


Put double scenarios on instances of a Class.

mock.instance_of(User).valid? {false}

Block Syntax

script =
mock(script) do |m|
  m.system("cd #{RAILS_ENV}") {true}
  m.system("rake foo:bar") {true}
  m.system("rake baz") {true}

Argument Wildcard matchers


mock(object).foobar(1, anything)
object.foobar(1, :my_symbol)








mock(object).foobar(duck_type(:walk, :talk))
arg =
def arg.walk; 'waddle'; end
def; 'quack'; end

Invocation Amount Wildcard Matchers


mock(object).method_name(anything).times(any_times) {return_value}

Special Thanks To

With any development effort, there are countless people who have contributed to making it possible. We all are standing on the shoulders of giants.

  • Pivotal Labs for sponsoring RR development

  • Parker Thompson for pairing with me

  • Felix Morio for pairing with me

  • Jeff Whitmire for documentation suggestions

  • Nick Kallen for documentation suggestion & bug reports

  • David Chelimsky for encouragement to make the RR framework, for developing the Rspec mock framework, and syntax ideas

  • Gerard Meszaros for his excellent book “xUnit Test Patterns”

  • Dan North for syntax ideas

  • Jim Weirich for developing Flexmock, the first Terse ruby mock framework in Ruby

  • James Mead for developing Mocha

  • Aslak Hellesoy for Developing Rspec

  • Stephen Baker for Developing Rspec

  • Dave Astels for some BDD inspiration

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