If this is a primary purpose of an ORM, then I wish I knew of one that isn't utterly failing at that.

One thing I've learned about this "impedance mismatch" is that it isn't a syntax thing, it's a fundamental difference in the way of viewing the world. The way you store data about the world is different from the way you model that world dynamically, with objects. I find it safer to always split out the "business model" from the storage layer, so that those different views don't interfere - and once you do that, you may as well implement the storage layer in a relational way.