After years of using relational databases, I've been learning MongoDB a bit recently, just out of curiosity. I can imagine the flexible structure being useful f

After years of using relational databases, I've been learning MongoDB a bit recently, just out of curiosity. I can imagine the flexible structure being useful for quick prototyping, or for knowledge representation research, or other particular instances where you really don't know exactly what you're going to need to store, and you don't need much in the way of the "relational" properties of a relational database. But for most real-world work... I find it hard to imagine preferring a document database.