I haven't used it myself, but I took a good look at the code for PeeWee for some changes I wanted to make to Django. It's incredibly well written, and weighs in at about 4000 LoC including 3 database specific dialects (pg, mysql, sqlite).

For anyone interested in how an ORM could be implemented, check it out: https://github.com/coleifer/peewee/blob/master/peewee.py