When should I use .lean() in Mongoose?
For read-only queries where you only need to send JSON. .lean() skips hydrating full Mongoose documents and returns plain JS objects. Faster and uses less memory. Do not use it if you need to call instance methods or save the document.
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More FAQs in Common Mongoose Mistakes (and How to Avoid Them)
No validation on updates, plain-text passwords, returning passwordHash, no indexes on hot queries, embedding unbounded arrays, N+1 queries, not handling duplicate key errors, no pagination, no .lean() for read-only queries, sync calls, mixing validation with logic, and no timestamps.
Mark the field with select: false in the schema. It is excluded from find queries by default. Use .select('+passwordHash') only when needed (e.g., for login).
Catch the error in your handler. Check err.code === 11000 (MongoDB duplicate key error). Return 409 with a clear message instead of letting the error handler send a 500.
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