Temporarily over-riding classes in ruby

method_override.rb

# Ever wondered, how RSpec does Stubbing.
# They use Ruby's metaprogamming abilities.
# Lets talk a closer look.

# Every class in ruby inherits from a special class
# named "Object" . 
class Object

  # We are adding a mock_methods function which will be available to every
  # object.
  def mock_methods(methods_to_mock)

    # We are backing-up the caller of this method
    original = self

    # We are creating a new class which will have the mock methods
    klass = Class.new(self) do

      # We are defining the mocked methods on this new class
      instance_eval do       
        methods_to_mock.each do |method, proc| 
          define_method(method, &proc)
        end            
      end
    end

    begin
      
      # Removing the original class
      Object.send(:remove_const, self.name.to_s)

      # Attaching the new class to the program
      Object.const_set(self.name.intern, klass)

      # Executing the block passed
      yield
    ensure

      # We are brining the original class back.
      Object.send(:remove_const, self.name.to_s)
      Object.const_set(self.name.intern, original)
    end
  end
end

class Duck
  def quak; puts "Quak";  end
end

Duck.new.quak #=> "Quak"

Duck.mock_methods(:quak => Proc.new { puts 'Wuff' }) do  
  Duck.new.quak #=> "Wuff"
end