Which isn't the best argument to make against MongoDB since you should have known - it's even part of their course curriculum - that map/reduce is not the optim

Which isn't the best argument to make against MongoDB since you should have known - it's even part of their course curriculum - that map/reduce is not the optimal way to aggregate in MongoDB. They have their own aggregation framework (https://docs.mongodb.com/manual/core/aggregation-pipeline/ https://docs.mongodb.com/manual/core/aggregation-pipeline/). I have no intention of defending MongoDB because what do I know, never worked with it in real life - but just out of curiosity I took the free courses they offer (https://university.mongodb.com/ https://university.mongodb.com/) and I find that a sizable share of the complaints about MongoDB come from people who don't seem to have learned much about the product they are using. It's like people complaining their new truck behaves badly in water. A lot of critics seem to have chosen MongoDB when they needed a SQL DB from day one. If you need full flexibility to (re)combine data you need SQL, for example. A document store isn't "schema-less" at all - much of the schema is built-in and very inflexible after that.