I also played around with a graph-document database hybrid when I had downtime, but never got it close to anything usable.

A json document database with relations between documents is basically a property graph. I've seen a lot of the document databases (rethinkdb, orientdb, elasticsearch, etc) that seem close to realizing this too, but no one has run with it.

Most document databases have some sort of nested "walker" api, and if your json doc has properties that are subdocuments, will walk those. That's basically a graph api.

I wrote it as a "streaming api" so a large document/property graph could be serialized out to the client as the lookup engine walked the graph, and you don't need to fully load a complex set of documents in the query layer memory before sending it out to the client.

But I just didn't have the development horsepower to get to the various query and index capabilities. I think the general distributed design was decent and offered hybrid plain-old-table, document, and graph capabilities all in one. And cassandra, PITA that it is, does linearly scale.