Everyone knows that you can have relations in MongoDB, right?

Relations are just _id of other objects from the same or another collection. In MongoDB, collections of objects are just like tables of rows in RDB.

Collections are just like tables with different columns on each row and the option to have nested tables.

Fun fact: In production, 99% of collections have the structure of a table. Unique "denormalization" in MongoDB is used only where it implements the unique task.