Testing railway-oriented business transactions with Rspec

Railway-oriented business transactions are a great way to unclutter your Rails controllers. We've already seen how to write 'em. Now let's see how we can test 'em. I'll be using dry-transaction as a business transaction DSL and RSpec for testing. I'm assuming that you already have RSpec set up for your app. If not, check Aaron Sumner's blog and his book Everyday Rails Testing With RSpec to saddle up.