What is the N+1 problem in Mongoose?
Looping over documents and calling findById for each reference does N+1 queries. Use populate to do it in 2 queries (one for the parent, one for all referenced docs).
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More FAQs in Mongoose Relationships: ref and populate Explained
Use a ref field that stores an ObjectId and references another model. Use populate on read to join the reference. Example: senderId: { type: ObjectId, ref: 'User' }, then Message.find().populate('senderId', 'firstName').
Use ref for one-to-many with large N (messages, connections, payments). Use embed for small, tightly coupled data (an address inside a user). Embedding large arrays bloats documents; use refs instead.
It does an extra query (or more for nested populate). For high-traffic endpoints, denormalize: store the sender's name on the message so you do not need to populate. Trade storage for query speed.
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