Jay Fields' Thoughts

The Mocha mocking framework provides a method, stub_everything, that returns an object that will respond 'nil' to any message sent to it. The stub_everything method is indispensable for creating an object that is needed for testing, but the interactions are completely unimportant.

For example, in the following test the ReservationService::reserve_for method needs to return a duck that responds to the same methods that are defined on the (not shown) Reservation class. However, the focus of the test has nothing to do with the behavior of the reservation duck, so stub_everything is the perfect duck because it requires no additional set up and responds to all the methods required without error.

require 'test/unit'
require 'rubygems'
require 'mocha'

class ReservationService
  # implementation
end
class MaidService
  # implementation
end
class VipService
  # implementation
end

class HotelRoom
  def book_for(customer)
    reservation = ReservationService.reserve_for(customer, self)
    MaidService.notify(reservation) if reservation.tomorrow?
    VipService.notify(reservation) if reservation.for_vip?
    reservation.confirmation_number
  end
end

class HotelRoomTests < Test::Unit::TestCase
  def test_book_for_reserves_via_ReservationService
    room = HotelRoom.new
    ReservationService.expects(:reserve_for).with(:customer, room).returns(stub_everything)
    room.book_for(:customer)
  end
end

The stub_everything method is also nice because it can take parameters. For example you may want to test that the book_for method returns a confirmation number. The following test specifies the value of the confirmation_number value, but it uses stub_everything so that each other method call will return nil (without causing an error).

require 'test/unit'
require 'rubygems'
require 'mocha'

class ReservationService
  # implementation
end
class MaidService
  # implementation
end
class VipService
  # implementation
end

class HotelRoom
  def book_for(customer)
    reservation = ReservationService.reserve_for(customer, self)
    MaidService.notify(reservation) if reservation.tomorrow?
    VipService.notify(reservation) if reservation.for_vip?
    reservation.confirmation_number
  end
end

class HotelRoomTests < Test::Unit::TestCase
  def test_book_for_reserves_via_ReservationService
    reservation = stub_everything(:confirmation_number => "confirmation number")
    ReservationService.stubs(:reserve_for).returns(reservation)
    assert_equal "confirmation number", HotelRoom.new.book_for(:customer)
  end
end

Another usage of stub_everything that might not be immediately apparent is that it can be used as both a stub and a mock. The following test creates a stub and then sets an expectation on it. As a result, the expectation is verified (which is the focus of the test) and all other calls to the same object simply do nothing.

require 'test/unit'
require 'rubygems'
require 'mocha'


class HotelTests < Test::Unit::TestCase
  def test_name_is_set_from_options
    hotel = stub_everything
    hotel.expects(:name=).with "Marriott"
    Hotel.stubs(:new).returns hotel
    Hotel.parse("Marriott||")
  end
end

class Hotel
  def self.parse(attributes)
    hotel = self.new
    hotel.name, hotel.location, hotel.capacity = attributes.split("|")
    hotel
  end
end

Using the stub_everything method of mocha can remove noise from tests and also make them more robust by ignoring all messages that are unimportant to the current test.

Labels: mocha, stub, testing, Testing Refactorings