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Partial Mock Object Or Mocking An Instance Of An Object.

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"What happens though, if you want to override a class in an instance of an object, and not all of its kind? Typically you would define a mock object, and create an instance of it. But, in Ruby there is an easier and faster way that doesn’t involve writing a different mock class for each different scenario – and it is made possible by the singleton class. This clever bit of ruby hackery lets you override the behaviour of a single instance of a class, creating what I’ve decided to call a partial mock object. To demonstrate, I’ve written a small method called override_method which will override the behaviour of the specified method in the passed object, like so:

# Overrides the method +method_name+ in +obj+ with the passed block
def override_method(obj, method_name, &block)
  # Get the singleton class/eigenclass for 'obj'
  klass = class <<obj; self; end 

  # Undefine the old method (using 'send' since 'undef_method' is protected)
  klass.send(:undef_method, method_name)

  # Create the new method
  klass.send(:define_method, method_name, block)

# Just an example class
class Foo 
  def do_stuff
    "I'm okay!" 

# Test code
list = []
5.times { list.push( }

# We override the method here!
override_method(list.first, :do_stuff) { "I'm NOT okay!" }

list.each_with_index { |f, i| puts "(#{i}) #{f.do_stuff}" }

(0) I'm NOT okay!
(1) I'm okay!
(2) I'm okay!
(3) I'm okay!
(4) I'm okay!
As you can see, only the first object in the array’s behaviour has been changed – the rest have remained untouched. Because of this, you can embed these partial dynamic mock objects deeply into your code without the need to specially instantiate a mock object deep in your code, or writing a ‘clever mock’ to only trigger the determined behaviour in certain objects.

Where this code comes in really handy is when you need an object to raise a difficult to simulate exception (like a disk full error) on a certain method to test your error handling – simply call override_method and pass in a call to raise and voila! Dynamic partial mock objects on the fly!"


Snippets Manager replied on Fri, 2006/08/25 - 9:09pm

It was doing things like this that led us to develop Mocha ( which allows you mock or stub methods on concrete objects or classes. In your example, instead of... override_method(list.first, :do_stuff) { "I'm NOT okay!" } ... you would write ... list.first.stubs(:do_stuff).returns("I'm NOT okay!") Where Mocha really comes into its own is doing this for classes e.g. Foo.stubs(:create).returns(mock_foo) In this case Mocha puts the class back together at the end of each test method, so other tests will be unaffected.