How to mock typeORM's getCustomRepository

When performing unit tests, it is often necessary to mock certain dependencies to isolate the code under test. When using TypeORM, the getCustomRepository function is used to retrieve custom repositories, which may involve database interactions. Therefore, it is common to mock it during testing.

Here are the steps to mock getCustomRepository in TypeORM:

Step 1: Create a Mock Repository

First, you need to create a mock repository class where you can override the methods in the repository as needed. For example, if you have a UserRepository class, you can create a mock class as follows:

typescript
class MockUserRepository {
  // Assuming the original method is find
  find() {
    // Return mock data or spy on the method as needed
    return Promise.resolve([{ id: 1, name: 'Alice' }]);
  }

  // ... Other methods can also be mocked here
}

Step 2: Mock getCustomRepository

In your test code, you need to mock the getCustomRepository function. Assuming you are using Jest as your testing framework, you can do the following:

typescript
import { getCustomRepository } from 'typeorm';

jest.mock('typeorm', () => {
  const actual = jest.requireActual('typeorm');
  return {
    ...actual,
    getCustomRepository: jest.fn(),
  };
});

Then, in your specific test cases, you can specify what getCustomRepository should return:

typescript
import { getCustomRepository } from 'typeorm';
import { UserRepository } from './UserRepository';

// Set up before each test case
beforeEach(() => {
  // Point the implementation of `getCustomRepository` to a mock `UserRepository` instance
  (getCustomRepository as jest.Mock).mockReturnValue(new MockUserRepository());
});

// Below is a specific test case
test('should do something with the mock repository', async () => {
  const userRepository = getCustomRepository(UserRepository);
  // Here, userRepository will be an instance of MockUserRepository
  const users = await userRepository.find();

  // Assertion
  expect(users).toEqual([{ id: 1, name: 'Alice' }]);
});

By doing this, you can control the behavior of getCustomRepository during testing without worrying about real database operations affecting your test results.

This is just an example; the specific mock implementation will depend on your testing framework and specific requirements. If you are using a different testing framework, the steps may vary, but the overall approach is similar: create a mock repository and replace getCustomRepository with one that returns your mock repository before running tests.