There are also some other (real) multi model databases, which have some great features & performance and are open source... ArangoDB and OrientDB to name to great ones. Anyone knows others?
Hardly a new idea:<p><a href="http://www.mgateway.com/docs/universalNoSQL.pdf" rel="nofollow">http://www.mgateway.com/docs/universalNoSQL.pdf</a>