test_helper.rb

class ActiveSupport::TestCase

  # Create a be_a / be_an matchy matcher.
  #   @video.should be_a(Video)
  #   @video.should_not be_an(Octopus)
  custom_matcher :be_a do |receiver, matcher, args|
    expected = args[0]
    matcher.positive_failure_message = "Expected #{ receiver.inspect } to be a #{ expected.inspect }."
    matcher.negative_failure_message = "Expected #{ receiver.inspect } to not be a #{ expected.inspect }."

    receiver.is_a? expected
  end

  alias :be_an :be_a
end