[awj]

I have no idea what the GP meant, but here's my take:

Set a minimal threshold of cyclomatic complexity. Below this threshold, function are assumed "so simple there are obviously no defects". Above this threshold you need tests, roughly prioritized in order of descending complexity.

It's not a bad idea, but that assumption of simplicity is a doozy. Also, figuring out cyclomatic complexity in Ruby (especially Rails apps) is ... hard. Especially if you want the computer to do it for you.