Prisma Kysely Generator

Common errors

• Error: Generator "kysely" could not be found.

  cause The `prisma-kysely` package is not installed, or the `provider` path in `schema.prisma` is incorrect.
  fix Install `prisma-kysely` using your package manager (`npm install prisma-kysely` or `bun add prisma-kysely`) and verify `generator kysely { provider = "prisma-kysely" }` in your `schema.prisma` file.

• TypeError: Type 'Decimal | null' is not assignable to type 'Decimal'.

  cause A custom scalar type, such as `Decimal` or `BigInt`, used in the Prisma schema is not correctly imported into the generated Kysely types file.
  fix In your `schema.prisma`, add a `banner` option to the `kysely` generator to include the necessary type import. Example: `banner = "import type { Decimal } from 'decimal.js';"`.

• Error: A database URL must be provided in your Prisma configuration.

  cause When upgrading to Prisma 7.0.0+, the `database url` must be moved from the `datasource` block to a `prisma.config.ts` file.
  fix Refer to the official Prisma 7.0.0 upgrade guide for instructions on configuring your database URL in `prisma.config.ts`.

• Error: Prisma Client is not compatible with your Prisma Schema.

  cause The installed `prisma-kysely` version or the Prisma Client version (`@prisma/client`) is not compatible with your Prisma schema's version requirements, often indicated by `prisma generate` failing.
  fix Ensure that your `prisma-kysely` package and all `@prisma/*` packages (e.g., `@prisma/client`, `@prisma/generator-helper`) are updated to compatible versions, typically aligning with the `prisma-kysely` major version's supported Prisma version.

Warnings

• breaking Upgrading to `prisma-kysely` v3.0.0 requires Prisma 7.0.0+ and Node.js >=22.x. Additionally, Prisma 7.0.0 introduced significant changes, including requiring `database url` to be moved from the `datasource` block to a `prisma.config.ts` file.
  Fix: Update your `prisma` dependency to `7.0.0` or later. Ensure your Node.js version is `22.x` or newer. Review the Prisma 7.0.0 upgrade guide for details on moving your database URL configuration to `prisma.config.ts`.
• breaking Upgrading to `prisma-kysely` v2.0.0 moved its supported Node.js version from 16 to 24 and required Prisma 6.10.1+.
  Fix: Ensure your Node.js environment is compatible with Node.js 24 and update your `prisma` dependency to `6.10.1` or later.
• gotcha If your Prisma schema utilizes custom scalar types (e.g., `Decimal`, `BigInt`) that are not native TypeScript types, you must use the `banner` configuration option to import their definitions into the generated Kysely type file. Failure to do so will result in TypeScript compilation errors due to unknown types.
  Fix: Add a `banner` configuration to your `kysely` generator in `schema.prisma`. For example: `banner = """import type { Decimal } from 'decimal.js';"""`.
• gotcha `prisma-kysely` requires a peer dependency on `prisma`. Mismatched versions between `prisma-kysely` and your installed `prisma` packages (`@prisma/client`, `@prisma/generator-helper`, `@prisma/internals`) can lead to generation failures or unexpected behavior.
  Fix: Always check the `prisma-kysely` release notes and `package.json` for the exact `prisma` peer dependency range and ensure your installed `prisma` packages fall within that range. Upgrade or downgrade `prisma` as needed.

Install

• npm install prisma-kysely npm
• yarn add prisma-kysely yarn
• pnpm add prisma-kysely pnpm

Imports

• DB
  wrong:

  import { DB } from '../src/db/types'; // 'type' keyword is important for tree-shaking

  correct:

  import type { DB } from '../src/db/types';

  This is an import for the *generated* Kysely database types. The path depends on your `output` and `fileName` configuration in `schema.prisma`.
• Kysely

  import { Kysely } from 'kysely';

  While `prisma-kysely` generates the types, Kysely itself is a separate library that you will install and use in conjunction with the generated types.
• generator kysely
  wrong:

  generator client { provider = "prisma-client-js" } // Not using prisma-kysely

  correct:

  generator kysely { provider = "prisma-kysely" }

  This is how you configure the `prisma-kysely` generator within your `prisma/schema.prisma` file, enabling it to generate the Kysely types.

Quickstart

This quickstart demonstrates how to configure `prisma-kysely` in your `schema.prisma` file, then initialize Kysely using the generated types for type-safe database queries. It includes an example of a type-safe SQL query for fetching a user and their post count.

import { Kysely, PostgresDialect } from 'kysely';
import { Pool } from 'pg';
import { config } from 'dotenv';
import type { DB } from './db/types'; // Adjust path based on your generator config

// Load environment variables from .env file
config();

// prisma/schema.prisma
/*
datasource db {
    provider = "postgresql"
    url      = env("DATABASE_URL")
}

generator kysely {
    provider = "prisma-kysely"
    output = "./src/db"
    fileName = "types.ts"
    enumFileName = "enums.ts"
    banner = """
import type { Decimal } from 'decimal.js';
"""
}

model User {
    id        String   @id @default(uuid())
    email     String   @unique
    name      String?
    posts     Post[]
    createdAt DateTime @default(now())
    updatedAt DateTime @updatedAt
    balance   Decimal? @db.Decimal(10, 2)
}

model Post {
    id        String   @id @default(uuid())
    title     String
    content   String?
    published Boolean  @default(false)
    author    User     @relation(fields: [authorId], references: [id])
    authorId  String
    createdAt DateTime @default(now())
    updatedAt DateTime @updatedAt
}
*/

// Configure the Kysely dialect (example for PostgreSQL)
const dialect = new PostgresDialect({
    pool: new Pool({
        connectionString: process.env.DATABASE_URL ?? 'postgresql://user:password@localhost:5432/mydb',
    }),
});

// Instantiate Kysely with your generated database types
export const db = new Kysely<DB>({
    dialect,
});

// Example usage: Fetch a user and their post count
async function getUserWithPostCount(userId: string) {
    try {
        const userWithPosts = await db
            .selectFrom('User')
            .selectAll('User')
            .leftJoin('Post', 'Post.authorId', 'User.id')
            .where('User.id', '=', userId)
            .select((eb) => eb.fn.count('Post.id').as('postCount'))
            .executeTakeFirst();

        if (userWithPosts) {
            console.log(`User: ${userWithPosts.name}, Email: ${userWithPosts.email}, Posts: ${userWithPosts.postCount}`);
            return userWithPosts;
        } else {
            console.log(`User with ID ${userId} not found.`);
            return null;
        }
    } catch (error) {
        console.error('Error fetching user with post count:', error);
        throw error;
    }
}

// To run this example, you'd need a DATABASE_URL env var and 'pg', 'dotenv' packages.
// Example call (replace with an actual user ID from your database):
// getUserWithPostCount('some-actual-user-id').catch(console.error);