Partial Mock Object or mocking an instance of an object. (See related posts)

From: http://www.karmiccoding.com/articles/2006/03/11/under-the-hood-with-ruby-partial-mock-objects-for-unit-testing

"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)
end

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

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

# 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}" }

Outputs:
(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!"