How To Mock Prisma With NestJS

Ethan Glover

Last Updated: 1 year ago

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.

1constructor(private readonly prisma: PrismaService) {}
2
3async create(
4  data: CreateOneUserArgs,
5  select?: Prisma.UserSelect
6): Promise<Partial<User>> {
7  return this.prisma.user.create({
8    ...data,
9    select,
10  });
11}

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

1import { PrismaClient } from '@prisma/client'
2import { mockDeep, mockReset, DeepMockProxy } from 'jest-mock-extended'
3
4import prisma from './client';
5
6jest.mock('./client', () => ({
7  __esModule: true,
8  default: mockDeep<PrismaClient>(),
9}))
10
11beforeEach(() => {
12  mockReset(prismaMock)
13})
14
15export const prismaMock = prisma as unknown as DeepMockProxy<PrismaClient>

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

1let service: UserService;
2
3beforeEach(async () => {
4  const module: TestingModule = await Test.createTestingModule({
5    providers: [
6      UserService,
7      { provide: PrismaService, useValue: prismaMock },
8    ],
9  }).compile();
10
11  service = module.get<UserService>(UserService);
12});

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.

1test('should create new user ', async () => {
2  const user = {
3    id: 1,
4    name: 'Jill',
5    email: 'hello@example.com',
6    acceptTermsAndConditions: true,
7  }
8
9  prismaMock.user.create.mockResolvedValue(user)
10
11  await expect(service.create(user)).resolves.toEqual({
12    id: 1,
13    name: 'Jill',
14    email: 'hello@example.com',
15  })
16})