>If you've ever heard of the ORM/Relational impedance mismatch, it's got nothing on the set of impedance mismatches between the way servers like to work, the HTTP protocol (and its still very page-based orientation in a world of streams), the browser's DOM model, and how Javascript works, especially if you want to get excellent performance out of it.

Mapping objects to tables is a solved problem. The only noteworthy challenge is subtyping but it has easy solutions. I'm surprised how well it works even if you have an old crusty database with an archaic table structure. Compared to serialising objects to JSON or other formats that have no concept of identity it's downright trivial. However mapping objects to tables is the primary thing an ORM really does. Usually it implements lazyness for correctness so that your code while inefficient still works as intended.

What an ORM however does not do is write queries for you. Databases are remote devices, you can't just treat remote objects as if they were local (like CORBA did) if you care about performance.

Remember: You still have to write your queries but usually the ORM still helps you writing queries by providing a query builder or has it's own query language. The point of the ORM is that you don't have to manually marshall rows into objects, it's not a tool to avoid queries. It's right there in the name: Object Relational Mapping. It does not say AutomaticQueryGenerator or something related to queries.