How To Mock Prisma With NestJS

When working with NestJS it can sometimes be difficult to figure out how to write tests around the dependency injection system. In searching for how I could write unit tests for services when using Prisma I found a lot of wild and hacky workarounds. But by consulting Prisma documentation I found a very simple, and clean answer.

First thing, let's say we have this code that creates a new user. You'll notice we're using dependency injection to bring in a PrismaService which connects to the database.

constructor(private readonly prisma: PrismaService) {}

async create(
  data: CreateOneUserArgs,
  select?: Prisma.UserSelect
): Promise<Partial<User>> {
  return this.prisma.user.create({
    ...data,
    select,
  });
}

First we need to follow the Prisma documentation to create mock objects. Most importantly for our case is creating the singleton.ts file.

import { PrismaClient } from '@prisma/client'
import { mockDeep, mockReset, DeepMockProxy } from 'jest-mock-extended'

import prisma from './client';

jest.mock('./client', () => ({
  __esModule: true,
  default: mockDeep<PrismaClient>(),
}))

beforeEach(() => {
  mockReset(prismaMock)
})

export const prismaMock = prisma as unknown as DeepMockProxy<PrismaClient>

We can then use the exported prismaMock as a provider in our unit test.

let service: UserService;

beforeEach(async () => {
  const module: TestingModule = await Test.createTestingModule({
    providers: [
      UserService,
      { provide: PrismaService, useValue: prismaMock },
    ],
  }).compile();

  service = module.get<UserService>(UserService);
});

Then just return to the Prisma documentation to keep following their example to write a test for creating a new user with the singleton method.

test('should create new user ', async () => {
  const user = {
    id: 1,
    name: 'Jill',
    email: 'hello@example.com',
    acceptTermsAndConditions: true,
  }

  prismaMock.user.create.mockResolvedValue(user)

  await expect(service.create(user)).resolves.toEqual({
    id: 1,
    name: 'Jill',
    email: 'hello@example.com',
  })
})