I don't know what kind of project it is - but have you actually considered just using Postgres? Its not hip and takes a bit to setup, but after you are done, you have reliable workhorse with all the stuff you need (access control, schemas, etc. pp.) It has a vast amount of documentation and reading about its advanced features never gets old. There is a lot of talent around that knows how to use it. If you still consider Ruby but don't like ActiveRecord magic, don't forget to have a look at Sequel.

I would recommend against DataMapper. It is nicely constructed, but has some weird issues. Also, DataMapper 1 is going to be replaced by DM 2, which is a completely different beast (but better, hopefully).

Don't be fooled by ease of setup of the DB too much. It should be rather straight-forward, but even if a DB can just be downloaded and ran to dabble with it, always remember that this just shifts the moment at which you really have to dig into the details. You don't want to do that 2 days before going to production. Be on the lookout for red flags, though: unreasonable and weird configurations that have to be flipped for no understandable reasons at all before going into production. That shows that the project is not maintained very well or the developers have lost track. Also try to figure out how long it takes you to solve a new problem with the database, with documentation reading and all. Pick the one that you can wrap your head around best.