Close
Glad You're Ready. Let's Get Started!

Let us know how we can contact you.

Thank you!

We'll respond shortly.

LABS
Introducing RR

This post was originally written by Brian Takita.

I’m pleased to introduce a new Test Double (or mock) framework named RR, which is short for Double Ruby.

Why a Double framework and not a Mock framework?

A mock is a type of test double. Since RR supports mocks, stubs, and proxies, it makes sense to refer to RR as a double framework. The proxy is a new usage pattern that I will introduce later in this article, and in more detail in future articles.

Unfortunately, the terminology over doubles has been contradictory depending on the framework. RR’s terminology tries to be as faithful as possible to Gerald Meszaros’ definition of test doubles. You can read more about test doubles in XUnit Test Patterns and Martin Fowler’s article, Mocks aren’t Stubs. Regretfully, this does mean that RR will have slightly different terminology than other double frameworks.

How does RR compare to other Mock frameworks?

Most double frameworks focus mainly on mocks (hence the categorization “mock framework”). RR’s focus is on enabling more double test patterns in a terse and readable syntax.

RR also does not have dedicated mock objects. It primarily uses the technique called ‘double injection’. Names that other frameworks use are ‘stub injection’, ‘mock object injection’, ‘partial mocking’, or ‘stubbing’. The term I’ll use for this is a double injection, since one or many doubles are being injected into an object’s method.

I’ll use trivial Rails examples to highlight the syntactical differences between RR, Mocha, Rspec’s mocks, and Flexmock. They may or may not be appropriate situations for mocks. The right situations for mocks is an entirely different discussion.

If there is better way to do any of the examples, please post a comment and I will gladly replace it.

Mocks

Here are the ways to mock the User.find method. The expectation is the User class object will receive a call to #find with the argument ’99’ once and will return the object represented by the variable user.

RR
mock(User).find('99') { user }

Mocha
User.expects(:find).with('99').returns(user)

spec/mocks
User.should_receive(:find).with('99').and_return(user)

Flexmock
flexstub(User).should_receive(:find).with('99').and_return(user).once

Stubs

Here are the ways to stub the User.find method. When the User class object receives a call to find with the argument ’99’ it will return user1. When User receives find with any other arg, it returns user2.

RR
stub(User).find('99') { user1 }

stub(User).find { user2 }

Mocha
User.stubs(:find).with(anything).returns(2)

User.stubs(:find).with('99').returns(1)

spec/mocks
users = {

  '99' => user1,

  'default' => user2

}

User.stub!(:find).and_return do |id|

  users[id] || users['default']

end

Flexmock
users = {

  '99' => user1,

  'default' => user2

}

flexstub(User).should_receive(:find).and_return do |id|

  users[id] || users['default']

end

Proxy

A proxy used with a mock or stub causes the real method to be called. Expectations can be placed on the invocation and the return value can be intercepted. The main rationales are test clarity and you can ensure that the methods are being called correctly, even after you refactor your code. I will delve more into proxies and their usage patterns in my next article.

Mock Proxy

The following examples set an expectation that User.find(’99’) will be called once. The actual user is returned.

RR
mock.proxy(User).find('99')

Mocha

You cannot implement this in Mocha. You can do an approximation in this situation however. This technique is not always the solution you need, though.

user = User.find('99')

User.expects(:find).with('99').returns(user)

spec/mocks
find_method = User.method(:find)

User.should_receive(:find).with('99').and_return(&find_method)

Flexmock
find_method = User.method(:find)

User.should_receive(:find).with('99').and_return(&find_method)

Stub Proxy

The following examples intercept the return value of User.find(’99’) and stub out valid? to return false.

RR
stub.proxy(User).find('99') do |user|

  stub(user).valid? {false}

  user

end

Mocha

Again, this is an approximation, since you cannot use proxies in Mocha.

user = User.find('99')

user.stubs(:valid?).returns(false)

User.stubs(:find).with('99').returns(user)

spec/mocks
find_method = User.method(:find)

User.stub!(:find).with('99').and_return do |id|

  user = find_method.call(id)

  user.stub!(:valid?).and_return(false)

  user

end

Flexmock
find_method = User.method(:find)

flexstub(User).should_receive(:find).with('99').and_return do |id|

  user = find_method.call(id)

  flexstub(user).should_receive(:valid?).and_return(false)

  user

end

instance_of

instance_of is method sugar than allows you to mock or stub instances of a particular class. The following examples mock instances of User to expect valid? with no arguments to be called once and return false.

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

Mocha
User.any_instance.expects(:valid?).returns(false)

spec/mocks
new_method = User.method(:new)

User.stub!(:new).and_return do |*args|

  user = new_method.call(*args)

  user.should_receive(:valid?).and_return(false)

  user

end

Flexmock
new_method = User.method(:new)

flexstub(User).should_receive(:new).and_return do |*args|

  user = new_method.call(*args)

  flexmock(user).should_receive(:valid?).and_return(false)

  user

end

More to come

This concludes the introduction to RR. RR enables some techniques, like proxying, that will make your tests clearer and less brittle. In the next article I will describe into patterns and techniques that will make mocks a more feasible tool for more situations.

Comments
  1. Carl says:

    The introduction of different mocking/stubbing strategies that RR implements is quite refreshing and I can certainly appreciate its terseness. I’m looking forward to your next post and am particularly interested in how RR can be used to stub out instances of unimplemented classes.

  2. Kamal Fariz says:

    How do the last two examples for `spec/mocks` and `flexmock` stub out the `valid?` call on any instance?

  3. Brian Takita says:

    Kamal: Thank you. I fixed the code samples.

    The mocking needs to happen inside of the block.

  4. Jay Fields says:

    What if the method expects a block?

    In the example you have mocking and stubbing specifying their return values in the block:

    mock(User).find(’99’) { user }
    stub(User).find(’99’) { user1 }
    stub(User).find { user2 }

    but, how would I stub a call to each, select, collect, etc.

    array = [1,2]
    mock(array).select …?

    Blocks have been a pain point for Mocha also, so I’m curious what the RR plan is.

  5. Jay Fields says:

    Also, the mocha stubbing example can be done (unless I’m missing some part of the example)

    require ‘rubygems’
    require ‘mocha’
    require ‘dust’
    require ‘test/unit’

    unit_tests do
    test “changing stub behavior” do
    User = Class.new
    User.stubs(:find).with(anything).returns(2)
    User.stubs(:find).with(’99’).returns(1)
    assert_equal 1, User.find(’99’)
    assert_equal 2, User.find
    end
    end

  6. Jay Fields says:

    Sorry, better formatting this time (hopefully):

    require ‘rubygems’
    require ‘mocha’
    require ‘dust’
    require ‘test/unit’

    unit_tests do
    test “changing stub behavior” do
    User = Class.new
    User.stubs(:find).with(anything).returns(2)
    User.stubs(:find).with(’99’).returns(1)
    assert_equal 1, User.find(’99’)
    assert_equal 2, User.find
    end
    end

  7. Brian Takita says:

    Jay: RR provide the yields method. In addition, the return block can also can also take the block convented to a Proc as the last argument (like spec/mocks).

    The array is a strange example, because in most situations you can depend on the real implementation for its methods.

    Lets say you do want to have something on the yield.
    Here is an example using yields:

    array = [1, 2]
    mock.proxy(array).select.yields(2)
    return_value = array.select do |value|
    value == 2
    end
    assert_equal [2], return_value

    Of course its limited to one call. It will not iterate. The block only gets called once. This example demonstrates:

    array = [1, 2]
    mock.proxy(array).select.yields(1)
    return_value = array.select do |value|
    value == 2
    end
    assert_equal [], return_value

    You can also use yields to set the return value.

    mock(array).select.yields(2) {[2]}

    Here is an example using the return value block:

    array = [1, 2]
    dup_array = array.dup
    mock(array).select do |block|
    current_iteration_value = dup_array.shift
    dup_array.select(&block)
    end
    return_value = array.select do |value|
    value == 2
    end
    assert_equal [2], return_value

    Blocks are tough to mock. What sort of requirements do you have?

  8. Brian Takita says:

    Jay: Thanks for the Mocha update. I’ll fix the example.

  9. Tammer Saleh says:

    This looks great. Without having known the terminology, I’ve been wishing that Mocha could setup an expectation that still passed the method call back to the original object. Proxying will definitely come in handy.

  10. Tammer Saleh says:

    Yeah, the all in one line proxying would be handy, but in Mocha specs I’ve just added one more line aliasing a method before stubbing it, and then returning the call on that, as needed (where the proposed preset reference solution wasn’t appropriate). It might not be quite as flexible as true proxying, but since I’m mocking everything out anyways, the fixed values are fine and it’s worked well enough when I’ve needed.

  11. Brian Takita says:

    Tammer: Can you provide a code example?

  12. Sergey Pariev says:

    Hello Brian.
    Today I’ve updated rails to rev 9248 and rr mocks for dynamic find_by methods in my specs ceased to work, instead giving the message

    undefined method `find_by_id’ for class `Class’
    /usr/lib/ruby/gems/1.8/gems/rr-0.4.8/lib/rr/double_injection.rb:17:in `alias_method’
    /usr/lib/ruby/gems/1.8/gems/rr-0.4.8/lib/rr/double_injection.rb:17:in `__send__’
    /usr/lib/ruby/gems/1.8/gems/rr-0.4.8/lib/rr/double_injection.rb:17:in `initialize’
    ….

    Could you please look into it ? Sorry if this page isn’t appropriate place to report rr bugs, I haven’t found links to bugtrackers here.

    Thanks, Sergey.

  13. Brian Takita says:

    Sergey, thanks for the bug report.

    The current bugtracker is at
    http://rubyforge.org/tracker/?atid=14084&group_id=3656&func=browse

    I’ll set up a rubyforge project for RR.

  14. Brian Takita says:

    Sergey, unfortunately I’m not able to reproduce this issue on Rails 2.1.0. Can you paste your test on http://pastie.caboo.se?

    Thanks,
    Brian

  15. James says:

    How do I call the actual method (or any method for that matter) on the class I’m trying to stub/mock/proxy?

    Example:

    `
    class MyObj
    def self.meth1; end
    def self.meth2; end
    end

    mock.proxy(MyObj).meth1 { meth2 }
    `

    Also what if I want to proxy a method but so that the mocked proxy is called FIRST, which would allow me to intercept the call before it’s being made. The current mock.proxy() seems to call the original method first and then the proxied code. If I’d want the proxy to be called first, and perhaps decide in the proxy block if the real method should be called or not..how do I do that?

    Thanks..

  16. Brian Takita says:

    @James: Both use cases are interesting things that I have yet not considered.

    The block is not instance_exec’d, so:

    mock.proxy(MyObj).meth1 { meth2 }
    

    would not work.

    Here is a way to solve it.

    mock.proxy(MyObj).meth1 { MyObj.meth2 }
    

    Perhaps it should be instance_exec’d. I’ll play around with it.

    Regarding the method interception, you could do the following:

    original_meth1 = MyObj.method(:meth1)
    mock(MyObj).meth1 do
      # do something
      original_meth1.call
    end
    

    I’d be willing to add a more explicit hook if a nice api can be found.
    Maybe something like:

    mock.proxy(MyObj).meth1.before {# do something}.after {# This is the return value}
    

    Such a change would be additive, so:

    mock.proxy(MyObj).meth1 {# This is the return value}
    

    would still work.

  17. Jaime says:

    Great library thanks!

    Have a bit of a problem, when the unit test in question defines the “teardown” method, none of the RR related assertions work. Is this normal?

    For example, the following test “fails” correctly:

    class MunnyTest < ActiveSupport::TestCase include RR::Adapters::TestUnit test “testing” do x = Object.new mock(x).to_s { ‘foobar’ } assert_equal 1,1 end end But the following does not (it “passes”): class MunnyTest < ActiveSupport::TestCase include RR::Adapters::TestUnit def teardown super end test “testing” do x = Object.new mock(x).to_s { ‘foobar’ } assert_equal 1,1 end end

Post a Comment

Your Information (Name required. Email address will not be displayed with comment.)

* Copy This Password *

* Type Or Paste Password Here *