Multiple Database connections in Prisma

image taken from https://www.prisma.io/blog/prisma-migrate-dx-primitives

If you come across a situation where you have two databases and need to connect to the two databases with Prisma, here’s how to do it.

TLDR: Talk is cheap, show me the code

https://github.com/sagarPakhrin/prisma-multidatabase-demo

Why do you need multiple database connections?

You don’t typically need to use multiple databases. If you’re not sure why you probably don’t need it.

But you could be in a situation where you have an old database that’s maintained by a different API and you’re using a separate database for your new API and you need to use both databases, you can set up Prisma to do so.

Let’s get started

The files/folder naming conventions used in the post is not an ideal way, it’s done to keep things simple.

Start a new nestjs project

nest new prisma-multidatabase-demo
cd prisma-multidatabase-demo

Install and Initialize Prisma

npm install prisma --save-dev
npx prisma init

Once you run the init command, a Prisma folder with schema.prisma will be generated, and DATABASE_URL with default value will be added to the .env

Your project should look like this.

Create MySQL container

version: '3.8'
services:
  mysql:
    image: mysql:8
    ports:
      - '3306:3306'
    environment:
      MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
      MYSQL_DATABASE: ${MYSQL_DATABASE}
      MYSQL_USER: ${MYSQL_USER}
      MYSQL_PASSWORD: ${MYSQL_PASSWORD}
    volumes:
      - ./tmp:/var/lib/mysql

Add variables to .env

MYSQL_ROOT_PASSWORD=prisma
MYSQL_DATABASE=prisma
MYSQL_USER=prisma
MYSQL_PASSWORD=prisma
DATABASE_URL=mysql://prisma:prisma@localhost:3306/prisma

Prisma needs to have a shadow database for it to be able to run migrations safely so let’s create a shadow database and a second database to demonstrate multiple database connections with Prisma.

Create a db/init.sql

CREATE DATABASE IF NOT EXISTS `prisma-shadow`;
GRANT ALL ON `prisma-shadow`.* TO 'prisma'@'%';
CREATE DATABASE IF NOT EXISTS `prisma2`;
GRANT ALL ON `prisma2`.* TO 'prisma'@'%';

This will create two databases, and grant all privileges for our user prisma

Let’s update docker-compose.yml to set db/init.sql as entry point for our MySQL container. In your compose file, replace the previous volumes with

volumes:
      - ./tmp:/var/lib/mysql
      - ./db:/docker-entrypoint-initdb.d

Run the container with docker-compose up -d and start the development server with npm run start:dev

Set the first database connection

#prisma/schema.prisma
generator client {
  provider = "prisma-client-js"
}
datasource db {
  provider          = "mysql"
  url               = env("DATABASE_URL")
  shadowDatabaseUrl = env("SHADOW_DATABASE_URL")
}
model User {
  id   Int    @id @default(autoincrement())
  name String
}

To keep things simple we will only create a User model with id and name. Now let’s add a script to run the migration in our package.json

“migrate”: “npx prisma migrate dev”

And run the command npm run migrate This will generate a Prisma client inside node_modules/.prisma/client , generate migration files and create appropriate tables in our prisma database

Note: The node_modules/.prisma/client/schema.prismafile should be the same as prisma/schema.prisma

Install and generate Prisma Client

npm install @prisma/client

Create prisma.service.ts in src folder

Update the API to get the list of users

You can create a route to add users and once you do that, you will get the list of the users.

Set the second database connection

Create a schema.prisma inside a different folder

For the second connection, the output directory for the client needs to point to a different directory. We will place it inside node_modules/@internal/prisma/client

Generate migrations

Let’s add a new command in package.json to generate migrations for the second schema file

"schema2:migrate": "npx prisma migrate dev --schema ./prisma2/schema.prisma"

Add BLOG_DATABASE_URL to .env file

BLOG_DATABASE_URL=mysql://prisma:prisma@localhost:3306/prisma2

Now when you run npm run schema2:migrate a new folder @internal/prisma/client will be generated.

If you have an old database with data in it, you don’t need to generate migrations files. Instead of running migrate dev, you can generate Prisma client by running

npx prisma generate --schema ./prisma2/schema.prisma

Create Prisma service

We need to create a separate prisma.service that will connect with the second database. This service will be the same but the PrismaClient will be imported from the newly generated @internal/prisma/client instead of the default@prisma/client

Now let’s update the API to list the blogs using the second database connection

And there you have it. You now have connections to both databases with Prisma.

TIP:
If you’re running CI pipelines, you might come across an issue where @internal/prisma/client is not found. you need to add one more step after installing dependencies for instance in GitHub actions, you can add the following step after npm install, which will generate the @internal/prisma/client

- name: Generate partner-database prisma client
  run: npx prisma generate --schema ./prisma2/schema.prisma

Conclusion

You can use multiple databases with Prisma by creating different schema files and generating separate Prisma clients for each database.
If there’s a better way to do this, please leave some comments.