I know these kinds of tutorials are just for getting a flavor of a framework or paradigm. Nice so see it.

As someone who's used Mongo for nearly a decade, I always feel compelled to note: Mongo will NOT scale with relational data like this. Sure, it can handle a lot of records. It'll have fast basic inserts or queries. It can scale with replicaSets and sharding. But it is NOT designed for relational data. Doing relational data is so painful in Mongo, with any sort of scale. Save yourself so much migration headache by using a "regular" SQL database, until it doesn't work (rare).