The answer
Use populate on read: Post.findById(id).populate('authorId', 'firstName lastName'). Returns the post with the author object embedded. The client gets everything in one response. Always select specific fields.
Writing APIs With Relationships in MongoDB
How do I write an API that returns related data in MongoDB?
More FAQs in Writing APIs With Relationships in MongoDB
For lists with many relationships, fetch parents and referenced docs in parallel, return both, let the client connect them. Avoids N+1 and avoids large nested payloads. Example: { posts: [...], authors: [...] }.
When deleting a user, delete their posts and messages too. Use deleteMany for each related collection, then delete the user. Do it in a transaction if your MongoDB supports it, so a failure rolls back.
For high-traffic reads of rarely-changing data. Store the sender's name on the message so you do not need to populate. When the user's name changes, update their messages (or accept staleness for a while).
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