Currently mongodb calls are not supported in Next.js middleware because middleware edge runtime nature is not compatible with mongodb adapter for connection.

There are alternatives to approach this here: https://medium.com/@chxiuy/mongodb-in-nextjs-overcoming-the-edge-runtime-middleware-hurdle-4beee31eaa30 and an official discussion about it here: https://github.com/vercel/next.js/discussions/46722