[spriggan3]

Mongo doesn't solve the impedance mismatch either, if you have 2 classes which have a many to many relationship you'll still need to think about it when you model datas, without any guarantee of consistency. Mongo DB barely make queries easier to write, without the power of SQL. How would you persist a "recursive" model with MongoDB while being able to do aggregation operations on the collection , like counting the number of models ? if you use a single collection for all the models, you'll then have to load all the models in the application code and count them in the code, where a simple COUNT would do the trick. With Mongo you're constently trying to reinvent SQL in the code. On the other hand, SQL allows one to write recursive queries for tree like structures.

I understand the trade offs in order to scale, but you can give up on joins WHEN it's time to scale. Mongo doesn't give one the choice at first place.