https://www.prisma.io/docs/guides/v7/database/multiple-databases#41-populate-your-databases-with-data

I wanted to use Prisma for two databases: a main database (PosrgreSQL) and test database (sqlite). Prisma did not support handling multiple databases with single schema, so I had to create two schemas and their configurations. prisma/main/schema.prisma: https://pastebin.com/keyQ6H8A prisma/test/schema.prisma: https://pastebin.com/5MPu0NAp

prisma/main/prisma.config.ts:

import "dotenv/config";
import { defineConfig, env } from "prisma/config";

export default defineConfig({
    schema: "./schema.prisma",
    migrations: { path: "./migrations" },
    datasource: { url: env("MAIN_DATABASE_URL") }
});

prisma/test/prisma.config.ts:

import "dotenv/config";
import { defineConfig, env } from "prisma/config";

export default defineConfig({
    schema: "./schema.prisma",
    migrations: { path: "./migrations" },
    datasource: { url: env("TEST_DATABASE_URL") }
});

I generated and instantiated Prisma clients. prisma/main/client.ts:

import { PrismaPg } from "@prisma/adapter-pg";

import { PrismaClient } from "./generated/client";
import { MAIN_DATABASE_URL } from "../../global/env";

const connectionString = MAIN_DATABASE_URL;

const adapter = new PrismaPg({ connectionString });
const client = new PrismaClient({ adapter });

export default client;

prisma/test/client.ts:

import { PrismaBetterSqlite3 } from "@prisma/adapter-better-sqlite3";

import { PrismaClient } from "./generated/client";
import { TEST_DATABASE_URL } from "../../global/env";

const connectionString = TEST_DATABASE_URL;

const adapter = new PrismaBetterSqlite3({ url: connectionString });
const client = new PrismaClient({ adapter });

export default client;

I have a authentication service which creates a user in the database. services\auth.service.ts:

import { hashSync } from "bcrypt";

import LOG_MESSAGES from "../global/logMessages";
import logger from "../global/logger";
import { IRegister } from "../interfaces";
import { userMapper } from "../mappers";
import { PrismaClient } from "../prisma/main/generated/client";

// 4 Authentication & authorization
export default class AuthService {
    client: PrismaClient;

    constructor(client: PrismaClient) {
        this.client = client;
    }

    // Main
    // Creates a new user in the Database. IRegister is interface
    async create(data: IRegister) {
        delete data.password2;
        data.email = data.email.toLowerCase();
        data.password = hashSync(data.password, 10);

        // 1 Parametrized queries
        const user = await this.client.user.create({ data });

        // 5 Security logging
        logger.info(LOG_MESSAGES.authServ.create, { userId: user.id });
        return userMapper.toClass(user);
    }
}

I tried condensing the client classes into one. prisma/client.ts:

import mainClient from "./main/client";
import { PrismaClient as MainClient } from "./main/generated/client";
import testClient from "./test/client";
import { PrismaClient as TestClient } from "./test/generated/client";

type Client = MainClient | TestClient;

export { Client, mainClient, testClient };

But then create function became uncallable.

// ...
import { Client } from "../prisma";
import { userMapper } from "../mappers";

// 4 Authentication & authorization
export default class AuthService {
    client: Client;

    constructor(client: Client) {
        this.client = client;
    }

    // Main
    async create(data: IRegister) {
        delete data.password2;
        data.email = data.email.toLowerCase();
        data.password = hashSync(data.password, 10);

        // 1 Parametrized queries
        const user = await this.client.user.create({ data });

This expression is not callable.
  Each member of the union type '((args: SelectSubset>) => Prisma__UserClient, T, { ...; }>, never, DefaultArgs, { ...; }>) | ((args: SelectSubset<...>) => Prisma__UserClient<...>)' has signatures, but none of those signatures are compatible with each other.

How do I use the same service for multiple databases?