Crux is an open-source document database with bitemporal graph queries. Most database systems are temporal, meaning they help us model facts along with the time at which they occurred. Bitemporal database systems let you model not just the valid time the fact occurred but also the transaction time when it was received. If you need a document store with graph capabilities for querying the content, then give Crux a try. It's currently in alpha and lacks SQL support, but you can use a Datalog query interface for reading and traversing relationships.