This is what I don't understand. For me MongoDB and PostgreSQL aren't really playing in the same field. Yes they are both databases. But I wouldn't use Postgre

This is what I don't understand. For me MongoDB and PostgreSQL aren't really playing in the same field. Yes they are both databases. But I wouldn't use PostgreSQL for document storage (the recent JSON addition is pretty weak). Likewise I wouldn't use MongoDB when I need my data to be more structured. I would have no issue using both at the same time. I guess everyone is looking for that mythical single stack of technologies they can use for every use case.