[joepie91_]

Migrations don't actually take more time than defining the schema implicitly in your application, and you don't even need to think about joins at all if it's only going to be a prototype.

MongoDB is far from "simple to work with". This is 100% marketing bullshit (along with dubious claims like it being "fast" when it never has been), trotted out by MongoDB's marketing department. Let's not forget that it has always been a commercial project.

EDIT: And to clarify, relational databases and document stores are not interchangeable. You need to pick the one that reflects your data model most accurately.

Usually that's going to be "relational", sometimes it will be "documents" - but even if it is, there are better options than MongoDB.