MongoDB Data Relationships

A customer has one profile, places many orders, and reviews many products. Each of these connections between documents is a relationship, and it is named after how many documents can appear on each side: one-to-one, one-to-many, or many-to-many.

A relational database enforces such connections with foreign keys and usually reads them with joins. MongoDB has no foreign keys, although it can join collections with the $lookup aggregation stage. You choose how to represent each relationship, either by embedding one document inside the other or by storing a reference