I like rethinkdb and have been successfully using their official ruby and js libraries for some time.
Nobrainer orm wasn't fun though, too many edge cases that interfere with activerecord and rails conventions. Going a bit on a tangent, after many experiments I've developed a strong conviction that pg is the best database choice for rails, especially with the jsonb datatype included in 9.4. It is the best of two worlds: reliable, proven sql db that plays really well with Rails and has nosql capabilities, including indexing and quering. So good. Ymmv.