How to Set Up a Prisma Client That Uses MongoDB as a Second Database

27 August 2023
prisma,
nextjs

I recently moved subtitle file storage in the Recos project to MongoDB. The frontend uses Next.js and Prisma, so I needed to add MongoDB support to the Prisma setup. I ran into a few small issues and wanted to record the solution here.

A Prisma schema does not connect to multiple databases at once. The approach is to create a separate file named mongo_schema.prisma in the prisma directory.

generator client {
  provider = "prisma-client-js"
  output   = "./generated/mongo_client"
}

datasource db {
  provider = "mongodb"
  url      = env("MONGO_URL")
}

model subtitle {
  id_            String @id @default(auto()) @map("_id") @db.ObjectId
  ...
}

The important part is the output field in the client generator. First add MONGO_URL to the project's .env file, then run prisma generate --schema=prisma/mongo_schema.prisma. This generates the client code inside the prisma directory.

Import it as follows, and it is ready to use:

import { PrismaClient as PrismaMongoClient } from "../prisma/generated/mongo_client";

One thing to keep in mind when connecting to MongoDB is the connection string's authentication database. The import used for this client is:

import { PrismaClient as PrismaMongoClient } from "../prisma/generated/mongo_client";

If the connection fails, try appending ?authSource=admin to MONGO_URL in the environment file.