Relationships between documents

The two basic options are embedding data and referencing other documents.

Embedding: data that's read together lives in one document, e.g. a delivery address stored in the order.

Reference: I store the ID of the related document, e.g. customerId, when the data has its own lifecycle or many relationships.

To be precise: $lookup lets you join data in an aggregation. populate in Mongoose is a library feature, not an automatic equivalent of a database foreign key.

Pitfall: embedding simplifies reads but can duplicate data. References need extra fetching and care about consistency.