Here is where I am at right now: I know that relational data, like that found in relational databases like MySQL or Postgres, is relational because there are relations between the tables. That is the reason why RDBMS have rigid schemas. On the other hand, NoSQL databases offer a looser schema but lose those relations. But MongoDB, for instance, offers official support (or at least blog posts) for associations between data (One-To-One, One-To-Many, etc).
I am really confused. Everyone keeps going on and on about the MERN/MEVN/MEAN stack and how awesome MongoDB is with Node which I agree and I love it, but sometimes people tend to jump onto the hype train too much. Can you give me your thoughts on this one?