Jest spy fails for typeorm repository update

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04-10-2024

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Jest Spy Fails for TypeORM Repository Update: Unmasking the Mystery

Problem: You're using Jest to test your TypeORM code, specifically an update operation in a repository. But, your Jest spy isn't capturing the update call as expected. Frustrating, right? Let's break down why this happens and how to fix it.

Scenario: Imagine you have a UserRepository with an update method that updates a user entity in your database. You want to test if the update method correctly calls the update method on the TypeORM Repository instance.

// UserRepository.ts
import { Injectable } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { User } from './user.entity';

@Injectable()
export class UserRepository {
  constructor(
    @InjectRepository(User)
    private readonly userRepository: Repository<User>,
  ) {}

  async update(userId: number, userDto: Partial<User>): Promise<User> {
    return this.userRepository.update(userId, userDto);
  }
}

// user.service.spec.ts
import { Test, TestingModule } from '@nestjs/testing';
import { UserRepository } from '../user.repository';
import { User } from '../user.entity';

describe('UserService', () => {
  let userService: UserService;
  let userRepository: UserRepository;

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

    userService = module.get<UserService>(UserService);
    userRepository = module.get<UserRepository>(UserRepository);
  });

  it('should update a user', async () => {
    const userId = 1;
    const userDto = { name: 'John Doe' };
    const updatedUser = { ...userDto, id: userId };

    jest.spyOn(userRepository, 'update').mockResolvedValue(updatedUser);
    const result = await userService.updateUser(userId, userDto);

    expect(result).toEqual(updatedUser);
    // This assertion fails:
    expect(userRepository.update).toHaveBeenCalledWith(userId, userDto); 
  });
});

The Mystery Unveiled:

The issue lies in the way TypeORM's Repository works. When you call userRepository.update(), it doesn't directly execute the SQL update query. Instead, it builds an entity update operation and stores it in an internal queue. This queue is then processed later, often when you commit changes to your database.

The Solution:

The key is to force TypeORM to process the queued update operation before asserting the spy call. You can do this by explicitly calling userRepository.manager.save(updatedUser). This triggers the update query and ensures your spy correctly captures the call.

// user.service.spec.ts (updated)
import { Test, TestingModule } from '@nestjs/testing';
import { UserRepository } from '../user.repository';
import { User } from '../user.entity';

describe('UserService', () => {
  // ... (rest of the code remains the same)

  it('should update a user', async () => {
    // ... (setup remains the same)

    jest.spyOn(userRepository, 'update').mockResolvedValue(updatedUser);
    const result = await userService.updateUser(userId, userDto);

    expect(result).toEqual(updatedUser);
    // Force TypeORM to execute the update
    await userRepository.manager.save(updatedUser); 
    // Now the assertion passes
    expect(userRepository.update).toHaveBeenCalledWith(userId, userDto); 
  });
});

Additional Insights:

• Understanding TypeORM's Query Execution: TypeORM uses a unit of work pattern, deferring database operations for efficiency. This is often desirable, but it requires an extra step when testing to ensure queries are actually executed.
• Alternative Solution: Instead of using save, you can use the flush() method of the EntityManager to force the execution of all pending operations, including updates.
• General Mocking Strategies: When mocking TypeORM repositories, consider the specific context. Sometimes, you might only need to mock the save method if you're testing the saving functionality, while other cases might require mocking specific methods like findOne or find.

Conclusion:

Testing TypeORM repositories effectively requires understanding how the library handles database interactions. By forcing the execution of update operations and understanding the internal mechanisms of TypeORM, you can write comprehensive and accurate unit tests for your code.