How do I invoke the debugger in a Rails functional test?

Question

With Rails 3, Ruby 1.9, and Test::Unit, what's the right way to invoke the debugger on a functional test? I've tried adding "debugger" and then running rake:test:functionals:

class AdminControllerTest < ActionController::TestCase
    test "should get index" do
        debugger
        get :index
        assert_redirected_to welcome_url
    end
end

but Ruby seems to ignore the call to debugger (the test goes ahead and runs normally). If I try running the test directly:

ruby -r debug test/functional/admin_controller_test.rb

then Ruby can't find test_helper.rb. If, following the answer below, I run:

ruby -I "lib:test" test/functional/admin_controller_test.rb

then it finds test_helper but again runs the test to completion without invoking the debugger. My Gemfile does have:

gem 'ruby-debug19', :require => 'ruby-debug', :group => :development

Answer 1

I've always been able to debug tests using just:

ruby -I"lib:test" test/functional/admin_controller_test.rb

I've got the following in my .rdebugrc file, if it helps:

set autolist
set autoeval
set autoreload
set forcestep