Framework to aid in handrolling mock/spy objects.
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Handrolling mocks is the best, but involves more overhead than necessary, and usually has less helpful error messages. Surrogate addresses this by endowing your objects with common things that most mocks need. Currently it is only integrated with RSpec.


  • Declarative syntax
  • Support default values
  • Easily override values
  • RSpec matchers for asserting what happend (what was invoked, with what args, how many times)
  • RSpec matchers for asserting the Mock's interface matches the real object
  • Support for exceptions
  • Queue return values
  • Initialization information is always recorded


Endow a class with surrogate abilities

class Mock
  Surrogate.endow self

Define a class method by using define in the block when endowing your class.

class MockClient
  Surrogate.endow self do
    define(:default_url) { '' }

MockClient.default_url # => ""

Define an instance method by using define outside the block after endowing your class.

class MockClient
  Surrogate.endow self
  define(:request) { ['result1', 'result2'] }
end # => ["result1", "result2"]

If you care about the arguments, your block can receive them.

class MockClient
  Surrogate.endow self
  define(:request) { |limit| { |i| "result#{}" } }
end 3 # => ["result1", "result2", "result3"]

You don't need a default if you set the ivar of the same name (replace ? with _p for predicates, since you can't have question marks in ivar names)

class MockClient
  Surrogate.endow self
  define(:initialize) { |id| @id, @connected_p = id, true }
  define :id
  define :connected?
end # => 12

Override defaults with will_<verb> and will_have_<noun>

class MockMP3
  Surrogate.endow self
  define :play # defaults are optional, will raise error if invoked without being told what to do
  define :info

mp3 =

# verbs
mp3.will_play true # => true

# nouns
mp3.will_have_info artist: 'Symphony of Science', title: 'Children of Africa' # => {:artist=>"Symphony of Science", :title=>"Children of Africa"}

Errors get raised

class MockClient
  Surrogate.endow self
  define :request

client =
client.will_have_request'Remote service unavailable')

rescue StandardError => e
  e # => #<StandardError: Remote service unavailable>

Queue up return values

class MockPlayer
  Surrogate.endow self
  define(:move) { 20 }

player =
player.will_move 1, 9, 3
player.move # => 1
player.move # => 9
player.move # => 3

# then back to default behaviour (or error if none provided)
player.move # => 20

You can define initialize

class MockUser
  Surrogate.endow self do
    define(:find) { |id| new id }
  define(:initialize) { |id| @id = id }
  define(:id) { @id }

user = MockUser.find 12 # => 12

RSpec Integration

Currently only integrated with RSpec, since that's what I use. It has some builtin matchers for querying what happened.

Load the RSpec matchers.

require 'surrogate/rspec'


Given this mock and assuming the following examples happen within a spec

class MockMP3
  Surrogate.endow self
  define(:info) { 'some info' }

Check if was invoked with have_been_asked_for_its

mp3.should_not have_been_asked_for_its :info
mp3.should have_been_asked_for_its :info

Invocation cardinality by chaining times(n)
mp3.should have_been_asked_for_its(:info).times(2)

Invocation arguments by chaining with(args) :title
mp3.should have_been_asked_for_its(:info).with(:title)

Supports RSpec's no_args matcher (the others coming in future versions)
mp3.should have_been_asked_for_its(:info).with(no_args)

Cardinality of a specific set of args with(args) and times(n) :title :title :artist
mp3.should have_been_asked_for_its(:info).with(:title).times(2)
mp3.should have_been_asked_for_its(:info).with(:artist).times(1)


Given this mock and assuming the following examples happen within a spec

class MockMP3
  Surrogate.endow self
  define(:play) { true }

Check if was invoked with have_been_told_to

mp3.should_not have_been_told_to :play
mp3.should have_been_told_to :play

Also supports the same with(args) and times(n) that nouns have.


Query with have_been_initialized_with, which is exactly the same as saying have_been_told_to(:initialize).with(...)

class MockUser
  Surrogate.endow self
  define(:initialize) { |id| @id = id }
  define :id
user = 12 == 12
user.should have_been_initialized_with 12


After you've implemented the real version of your mock (assuming a top-down style of development), how do you prevent your real object from getting out of synch with your mock?

Assert that your mock has the same interface as your real class. This will fail if the mock inherits methods which are not on the real class. It will also fail if the real class has any methods which have not been defined on the mock or inherited by the mock.

Presently, it will ignore methods defined directly in the mock (as it adds quite a few of its own methods, and generally considers them to be helpers). In a future version, you will be able to tell it to treat other methods as part of the API (will fail if they don't match, and maybe record their values).

class User
  def initialize(id)end
  def id()end

class MockUser
  Surrogate.endow self
  define(:initialize) { |id| @id = id }
  define :id

# they are the same
MockUser.should substitute_for User

# mock has extra method
MockUser.define :name
MockUser.should_not substitute_for User

# the same again via inheritance
class UserWithName < User
  def name()end
MockUser.should substitute_for UserWithName

# real class has extra methods
class UserWithNameAndAddress < UserWithName
  def address()end
MockUser.should_not substitute_for UserWithNameAndAddress

Sometimes you don't want to have to implement the entire interface. In these cases, you can assert that the methods on the mock are a subset of the methods on the real class.

class User
  def initialize(id)end
  def id()end
  def name()end

class MockUser
  Surrogate.endow self
  define(:initialize) { |id| @id = id }
  define :id

# doesn't matter that real user has a name as long as it has initialize and id
MockUser.should substitute_for User, subset: true

# but now it fails b/c it has no address
MockUser.define :address
MockUser.should_not substitute_for User, subset: true

How do I introduce my mocks?

This is known as dependency injection. There are many ways you can do this, you can pass the object into the initializer, you can pass a factory to your class, you can give the class that depends on the mock a setter and then override it whenever you feel it is necessary, you can use RSpec's #stub method to put it into place.

Personally, I use Deject, another gem I wrote. For more on why I feel it is a better solution than the above methods, see it's readme.

But why write this?

Need to put an explanation here soon. In the meantime, I wrote a blog that touches on the reasons.

Special Thanks

  • Kyle Hargraves for changing the name of his internal gem so that I could take Surrogate
  • David Chelimsky for pairing with me to make Surrogate integrate better with RSpec
  • Corey Haines for pairing on substitutability with me
  • Enova for giving me time and motivation to work on this during Enova Labs.
  • 8th Light for giving me time to work on this during our weekly Wazas, and the general encouragement and interest


  • Add a better explanation for motivations
  • Figure out whether I'm supposed to be using clone or dup for the object -.^ (looks like there may also be an initialize_copy method I can take advantage of instead of crazy stupid shit I'm doing now)
  • don't blow up when delegating to the Object#initialize with args
  • queues should not reset when done, they should raise

Future Features

  • have some sort of reinitialization that can hook into setup/teardown steps of test suite
  • Support arity checking as part of substitutability
  • Support for blocks
  • Ability to disassociate the method name from the test (e.g. you shouldn't need to change a test just because you change a name)
  • declare normal methods as being part of the API (e.g. for inheritance)
  • assertions for order of invocations & methods
  • class generator? (supports a top-down style of development for when you write your mocks before you write your implementations)
  • extract surrogate/rspec into its own gem?
  • deal with hard dependency on rspec-mocks
  • support subset-substitutabilty not being able to touch real methods (e.g. #respond_to?)