Unit Test Custom Repository in NestJS

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This is part 2 of Use TypeOrm Custom Repository in NestJS and this post is about unit testing custom repository.

In part one, I created a NestJS demo with controllers, services and custom repositories. The demo also added unit testing on controllers and services in which I mocked the methods of custom repositories with jest.fn(). The missing piece is unit testing on custom repository and I will fill the hole in this post.


Jest is an open source JavaScript testing framework that can use with Angular and NestJS. I used Jest to test controllers and services in part 1 and it can also be used to test custom repository.

The functions of the custom repository query database to obtain result set through TypeORM. However, there is no database running in custom repository testing and calling any repository method can lead to error. The solution is to mock the TypeORM functions such that they don’t call the database but the fake implementations to obtain test results. This is where Jest comes in because jest.fn() and jest.Spy().mockImplementation() are designed to do it.

Step 1: Create a spec file for user repository

A spec file is a Javascript/TypeScript file that encapsulates testing codes to validate the functionality of a class. It is important to import the necessary modules and providers to TestingModule before writing the first test case.

// user-repository.spec.ts

describe('UserRepository', () => {
  let service: UserRepository
  let now: Date

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [UserRepository],

    service = module.get<UserRepository>(UserRepository)
    now = new Date(Date.now())

  it('should be defined', () => {

Step 2: Write test cases to validate basic CRUD functions

Let remind the readers the implementation of UserRepository.

// user.repository.ts

export class UserRepository extends Repository<User> {
  async getUsers(): Promise<User[]> {
    return this.find({
      order: {
        name: 'ASC',
        lastname: 'ASC',

  async getUser(id: string): Promise<User | undefined> {
    return this.findOne(id)

  async createUser(newUser: NewUser): Promise<User> {
    return this.save(this.create(newUser))

  async getUserWithReports(id: string): Promise<User | undefined> {
    return this.createQueryBuilder('user')
      .leftJoinAndSelect('user.reports', 'reports')
      .where('user.id = :id', { id })

I consider getUsers, getUser and createUser basic CRUD functions because they are straightforward; calling TypeOrm functions directly to obtain results. getUserWithReports is complicated because it chains builder functions of TypeORM to compose a SQL query to obtain results from the database.

It also means less effort to mock straightforward TypeORM functions than builder functions.

Let’s write test case for getUsers function

describe('getUsers', () => {
    it('should find and return all users', async () => {
      const users: User[] = [
        { id: v4(), name: 'John', lastname: 'Doe', age: 10, createdAt: now, updatedAt: now, version: 1, reports: [] },
        { id: v4(), name: 'Jane', lastname: 'Doe', age: 22, createdAt: now, updatedAt: now, version: 1, reports: [] },
      jest.spyOn(service, 'find').mockImplementation(() => Promise.resolve(users))

      const result = await service.getUsers()
        order: {
          name: 'ASC',
          lastname: 'ASC',


  1. jest.spyOn(service, ‘find’).mockImplement() spies this.find() and provides a mock function that returns a promise that returns User array
  2. const result = await service.getUsers() returns the results of getUsers()
  3. expect(service.find).toBeCalled() ensures that this.find() is called within getUsers()
  4. expect(service.find).toBeCalledWith({ orders: { name: ‘ASC’, lastname: ‘ASC’ }}) ensures that this.find accepts expected argument
  5. expect(result).toEqual(users) ensures that the result is the same as the User array

I am not going to cover getUser() because it is similarly to testing getUsers()

Testing createUser() is similar to testing getUsers() but we have to mock two implementations: this.create and this.save

describe('createUser', () => {
    it('should create a new user', async () => {
      const id = v4()
      const newUser = {
        name: 'New',
        lastname: 'User',
        age: 21,

      const user: User = {
        name: 'John',
        lastname: 'Doe',
        age: 10,
        createdAt: now,
        updatedAt: now,
        version: 1,
        reports: [],
      jest.spyOn(service, 'create').mockImplementation(() => user)
      jest.spyOn(service, 'save').mockImplementation(() => Promise.resolve(user))

      const result = await service.createUser(newUser)


  1. jest.spyOn(service, ‘create’).mockImplement() spies this.create() and provides a mock function that returns a new user
  2. jest.spyOn(service, ‘save’).mockImplement() spies this.save() and provides a mock function that returns a promise that returns the new user
  3. const result = await service.createUser(newUser) returns the result of createUser()
  4. expect(service.create).toBeCalled() ensures that this.create() is called within createUser()
  5. expect(service.save).toBeCalled() ensures that this.save() is called within createUser()
  6. expect(service.create).toBeCalledWith(newUser) ensures that this.create accepts expected argument
  7. expect(service.save).toBeCalledWith(service.create(newUser)) ensures that this.create is passed to this.save()
  8. expect(result).toEqual(user) ensures that the result is the same as the new user

Step 3: Write test case to validate getUserWithReports

// user.repository.ts

async getUserWithReports(id: string): Promise<User | undefined> {
    return this.createQueryBuilder('user')
      .leftJoinAndSelect('user.reports', 'reports')
      .where('user.id = :id', { id })

Testing getUserWithReports is tricky because the builder functions are chained together and finally getOne() returns an entity to service. Unlike this.find, this.createQueryBuilder does not return entity but returning a class/object with a leftJoinAndSelect function. Similarly, leftJoinAndSelect returns a class/object with a where function and where returns a class/object with getOne function. Once you understand what they do, you can easily mock them with jest.fn().

describe('getUserWithReports', () => {
    it('should return a user with reports', async () => {
      // deleted codes that initialize other objects

      const user: User = {
        reports: [report],

      const getOne = jest.fn(() => user)
      const where = jest.fn(() => ({
      const leftJoinAndSelect = jest.fn(() => ({
      const createQueryBuilder = () =>
        } as unknown as SelectQueryBuilder<User>)
      jest.spyOn(service, 'createQueryBuilder').mockImplementation(createQueryBuilder)

      const result = await service.getUserWithReports(id)
      expect(leftJoinAndSelect).toBeCalledWith('user.reports', 'reports')
      expect(where).toBeCalledWith('user.id = :id', { id })


  1. const getOne = jest.fn(() => user) is a mock function that returns a user with reports
  2. const where = jest.fn(() => ({ getOne })) is a mock function that returns the getOne function
  3. const leftJoinAndSelect = jest.fn(() => ({ where })) is a mock function that returns the where function
  4. jest.spyOn(service, ‘createQueryBuilder’).mockImplement(createQueryBuilder) spies this.createQueryBuilder() and provides a mock function that returns the leftJoinAndSelect function
  5. const result = await service.getUserWithReports(id) returns the result of getUserWithReports()
  6. expect(service.createQueryBuilder).toBeCalled() ensures that this.createQueryBuilder() is called
  7. expect(service.createQueryBuilder).toBeCalledWith(‘report’) ensures that this.createQueryBuilder(‘report’) is called
  8. expect(leftJoinAndSelect).toBeCalled() ensures that .leftJoinAndSelect() is called
  9. expect(leftJoinAndSelect).toBeCalledWith(‘user.reports’, ‘reports’) ensures that .leftJoinAndSelect(‘user.reports’, ‘reports’) is called
  10. expect(where).toBeCalled() ensures that .where() is called
  11. expect(where).toBeCalledWith(‘user.id = :id’, { id }) ensures that .where(‘user.id = :id’, { id }) is called
  12. expect(getOne).toBeCalled() ensures that .getOne() is called
  13. expect(result).toEqual(user) ensures that result is the same as user

Final thoughts

Testing is tedious because it involves experience and long hours of practices for developers to mock APIs and functions to allow test cases to perform. Test cases should validate that correct results are received, expected functions are invoked and expected arguments are passed to the functions. Otherwise, developers do not have high confidence that the quality of the software is good before it is shipped to quality assurers for quality assurance.

I hope this article helps developers who use NestJS to build their backend applications and need help to unit test resources (controllers, services and custom repositories).


  1. Repo: https://github.com/railsstudent/nest-poc-standard-version
  2. Clock-In/Out System Part 9: Back-End Testing — Unit Testing of Services