I think the main thing about using an ORM (and I believe that Rails' active record is among the very best ORMs out there) is that you should never just blindly let it run without inspecting the queries it produces.
I'm constantly monitoring the logs during development to make sure it's doing exactly what I want. And the odd time when it's not the answer is usually within ORM api itself and doesn't require me to shift to raw sql.
This is why I never understood the argument that ORMs are for people that don't know SQL (not to suggest you were making such an argument). If anything being proficient at SQL makes you that much better at handling the abstraction that the ORM provides and that much better at examining the resulting queries.
I'm constantly monitoring the logs during development to make sure it's doing exactly what I want. And the odd time when it's not the answer is usually within ORM api itself and doesn't require me to shift to raw sql.
This is why I never understood the argument that ORMs are for people that don't know SQL (not to suggest you were making such an argument). If anything being proficient at SQL makes you that much better at handling the abstraction that the ORM provides and that much better at examining the resulting queries.