That sounds like a good use for CouchDB; I'm doing something similar. CouchDB shines at that stuff (and as a bonus, avoids some of the issues the OP was having

That sounds like a good use for CouchDB; I'm doing something similar. CouchDB shines at that stuff (and as a bonus, avoids some of the issues the OP was having with MongoDB. CouchDB views aren't magic, but they're powerful and functional; more than capable of doing some basic joins).