Testing Rails Helper Classes

I’m making my way through Agile Web Development with Rails TDD style after an earlier sillier attempt at waiting until the testing chapter about 150 pages into the book. I really like the skeletons for unit tests and functional tests. Then I wanted to test the following helper method in the ApplicationHelper class:

def fmt_dollars(amount)
    sprintf("$%0.2f", amount)
  end

This resulted in a bit of thrashing for about 30 minutes without much luck until I came across a post by Doug Alcorn on Testing Rails Helpers. The solution looks like:

require File.dirname(__FILE__) + '/../test_helper'

class ApplicationHelperTest < Test::Unit::TestCase
  include ActionView::Helpers::TextHelper
  include ActionView::Helpers::TagHelper
  include ApplicationHelper
  # include whatever helpers you want to test here, sometimes you'll need
  # to include some of the Rails helpers, as I've done above.

  def test_formatting_dollars
	assert_equal "$29.90", fmt_dollars(29.9)
  end
end

Considering unit testing utility classes are some of the easiest things to test in Java I felt a bit incompetent when I had a tough time with this. I think I need to better understand how Ruby does packaging with modules and how the include keyword really works.