The answer
No. It is an index. It does not produce a ValidationError. It produces a 11000 duplicate key error. Catch it in your handler: if (err.code === 11000) return res.status(409).json({ error: 'Email already in use' }).
Schema Validation in Mongoose: Best Practices
Is unique: true a validator in Mongoose?
More FAQs in Schema Validation in Mongoose: Best Practices
Use built-in validators (required, min, max, minlength, maxlength, enum, match). Add custom validators with the validate property for complex rules. Use runValidators: true on updates. Combine with Zod at the middleware layer for defense in depth.
Mongoose runs validation on .save() and .create() but not on findByIdAndUpdate or updateOne by default. Pass { runValidators: true, new: true } on every update to enforce validation.
No. Validators should check data shape (user must be 18). Business rules (user cannot signup if they have unpaid invoices) belong in a service. Mixing them makes validators hard to test and reason about.
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