Relational DBs for everything that needs joins. MongoDB is horrible with that and should not be used. One would not put an off-road vehicle on a race track and

Relational DBs for everything that needs joins. MongoDB is horrible with that and should not be used. One would not put an off-road vehicle on a race track and expect it to win. I use MongoDB for everything where I don't know the data structure before hand. MongoDB is very good in indexing unknown data structures and makes them searchable. Also, when I build prototypes I often use MongoDB.