I have started using the better assert_difference that was posted at blog.caboo.se to improve my rails tests. This allowed me to write tests such as:

  assert_difference Item, :count do
    Item.create(:name => "Monkey magic")
  end

This replaces:

  item_count = Item.count
  Item.create(:name => "Monkey magic")
  assert_difference, item_count + 1, Item.count

but I also wanted to be able to test if emails were sent by writing:

  assert_difference ActionMailer::Base.deliveries, :size do
    SystemNotifier.deliver_important_message
  end

That didn’t work and it was noted on that website that the code didn’t work properly with arrays so I have modified the assert_difference method to check if the objects respond to the method (e.g. size) and act accordingly:

  def assert_difference(objects, method = nil, difference = 1)
    initial_values = []
    if objects.respond_to? method
      initial_values = objects.send(method)
    else
      objects = [objects].flatten
      initial_values = objects.inject([]) { |sum,obj| sum << obj.send(method) }
    end

    yield

    if difference.nil?
      objects.each_with_index { |obj,i|
        assert_not_equal initial_values[i], obj.send(method) #, "#{obj}##{method}"
      }
    else
      if objects.respond_to? method
        assert_equal initial_values + difference, objects.send(method) #, "#{objects}##{method}"
      else
        objects.each_with_index { |obj,i|
          assert_equal initial_values[i] + difference, obj.send(method) #, "#{obj}##{method}"
        }
      end
    end
  end

I have tested it for an array such as ActionMailer::Base.deliveries but I haven't tested it in other situations so hopefully it won't create any problems.

  def assert_difference(objects, method = nil, difference = 1)
    initial_values = []
    if objects.respond_to? method
      initial_values = objects.send(method)
    else
      objects = [objects].flatten
      initial_values = objects.inject([]) { |sum,obj| sum << obj.send(method) }
    end

    yield

    if difference.nil?
      objects.each_with_index { |obj,i|
        initial_values[i].should_not_equal obj.send(method) #, "#{obj}##{method}"
      }
    else
      if objects.respond_to? method
        (initial_values + difference).should_eql objects.send(method) #, "#{objects}##{method}"
      else
        objects.each_with_index { |obj,i|
          (initial_values[i] + difference).should_eql obj.send(method) #, "#{obj}##{method}"
        }
      end
    end
  end