Best practice for instantiating PrismaClient with Next.js

Problem

Lots of users have come across this warning while working with Next.js in development:

warn(prisma-client) There are already 10 instances of Prisma Client actively running.

There's a related discussion and issue for the same.

In development, the command next dev clears Node.js cache on run. This in turn initializes a new PrismaClient instance each time due to hot reloading that creates a connection to the database. This can quickly exhaust the database connections as each PrismaClient instance holds its own connection pool.

Solution

The solution in this case is to instantiate a single instance PrismaClient and save it on the globalThis object. Then we keep a check to only instantiate PrismaClient if it's not on the globalThis object otherwise use the same instance again if already present to prevent instantiating extra PrismaClient instances.

import { PrismaClient } from '@prisma/client'

const prismaClientSingleton = () => {
  return new PrismaClient()
}

declare global {
  var prisma: undefined | ReturnType<typeof prismaClientSingleton>
}

const prisma = globalThis.prisma ?? prismaClientSingleton()

export default prisma

if (process.env.NODE_ENV !== 'production') globalThis.prisma = prisma

You can extend Prisma Client using a Prisma Client extension by appending the $extends client method when instantiating Prisma Client as follows:

import { PrismaClient } from '@prisma/client'

const prismaClientSingleton = () => {
  return new PrismaClient().$extends({
    result: {
      user: {
        fullName: {
          needs: { firstName: true, lastName: true },
          compute(user) {
            return `${user.firstName} ${user.lastName}`
          },
        },
      },
    },
  })
}

After creating this file, you can now import the extended PrismaClient instance anywhere in your Next.js pages as follows:

// e.g. in `pages/index.tsx`
import prisma from './db'

export const getServerSideProps = async () => {
  const posts = await prisma.post.findMany()

  return { props: { posts } }
}