I think the point is, Active Record (like any orm) is a leaky abstraction that covers a small, but common percentage of what you can do with SQL. Any time I've run into problems fighting the "magic" of Active Record, my time was almost always better spent just taking back to hand written SQL (which isn't terrifically difficult with Rails).

Rails absolutely is a framework where you need to abandon some control and trust the framework though. If you need to know exactly how your abstractions work under the hood, it's a poor choice IMO. I doubt there a single person who has comprehensive knowledge of how every single part of Rails works.