What I was trying (and evidently failing) to say is that a sufficiently powerful query generation system that's designed for people who actually like their database to be able to generate the exact SQL they would've written by hand is essential to the 'mapper' part of the equation being able to smooth out any impedence mismatches between the appropriate object model and the appropriate database schema.
Hopefully that longer answer is a bit clearer than my first attempt.
SQLAlchemy is pretty close. Been quite a few years since I've had a chance to get drunk with the authors and compare notes though, so I'm not going to try and get into details because I'm pretty much guaranteed to get some of them wrong.
In http://p3rl.org/DBIx::Class perl has had such a thing for over a decade now.
It makes me cry that nobody's ever adequately cloned it into other languages. Eventually I'll probably do so myself.