Overview

This post walks through a real error I hit in production: duplicate key value violates unique constraint. We explore the stack trace, reproduction, and a robust fix.

Key points

- Validate first, write once patterns

- Database-level ON CONFLICT DO NOTHING

- Safe upserts with unique indexes

Code

// Example with PostgreSQL

await db.user.create({ email })

.catch(e => {

if (e.name === 'SequelizeUniqueConstraintError') {

return db.user.findOne({ where: { email } })

}

throw e

})

Takeaways

Prefer database constraints and idempotent endpoints over optimistic UI-only checks.