Create mocks & stubs on ActiveRecord instances by <id>
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Passport Control

Passport Control is a Rails plugin that augments ActiveRecord model classes with per-record callbacks. The intended purpose is to allow mocking & stubbing in tests on a per-record, rather than per-instance, basis.

Here is a typical example of mocking a method in a test setup: (*)

setup do
  @user = User.make
  @content = @user.contents.make
  mock( @content ).foo { "foo!" }

Setting up a mock in this way works fine in a context where you can pass your mocked @content instance directly into your test (e.g. a unit test). But this becomes more complicated when your test is not responsible for directly providing the object you need to mock.

For example controller methods usually take the id of a record and look up an instance using find and friends. Calls to find returns the same record but a different instance to the one your mock method is installed on.

Typical work arounds to this problem involve either any_instance (if it's available to you), or stubbing your model class to ensure your pre-mocked instance is returned by find. This can get messy, especially where associations are involved.

Maybe you've wished you could simply tell Rails "Look, just mock method #foo of record 13 wherever it comes from"? That's where Passport Control comes in.

Here's an example:

setup do
  @user = User.make
  @content = @user.contents.make
  @content.at_passport_control { |content| mock( content ).foo { "foo!" }
  get :show, :id =>

Calling at_passport_control on the instance ensures that the block is called when the record corresponding to @content gets instantiated by ActiveRecord, no matter what chain of methods is involved. This example ensures that when the controller method #show looks up the record given it's id that the returned instance correctly mocks the method #foo.

Test Framework Integration

To ensure that passport control mocks & stubs don't leak from one test to another install the PassportControl adapater. An adapter is provided for Test::Unit based systems, add the following to your test_helper.rb

require 'passport_control'

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

A similar adapter should be easy enough to write for other frameworks.

The Low Down

Passport Control is specifically intended for use in writing tests not for use in production code.

It works by replacing the instantiate method of ActiveRecord::Base with an augmented version that manages the per-class/per-id callback lists. Initial results suggest that this doesn't add a significant overhead during test runs but, given how often instantiate is going to get called during the lifetime of a real application, you don't want it in production.

Also monkeypatching such a fundamental ActiveRecord method is surely evil and angry villagers with torches and pitchforks will be involved somehow. You have been warned!