What is the N+1 problem and how do you fix it?
Looping over documents and calling findById for each reference does N+1 queries. Fix: use populate to do it in 2 queries (one for the parent, one for all referenced docs).
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Use refs (ObjectId fields with ref) and populate on read. For high-traffic reads, denormalize (copy key fields). For large N (messages, comments), use a separate collection with a ref. For small, tightly coupled data, embed.
Embed for small, tightly coupled data that always loads together (an address in a user). Reference for one-to-many with large N (messages, comments, payments). Embedding large arrays bloats documents; use refs.
Delete related documents in the same handler. Use deleteMany for each related collection, then delete the parent. Do it in a transaction if your MongoDB supports it, so a failure rolls back.
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