How Do You Mock EntityManger In TypeORM?

In TypeORM, mocking the EntityManager helps developers isolate database operations during unit tests, thereby improving the speed and efficiency of tests. To mock the EntityManager, you can typically use common JavaScript testing libraries such as Jest or Sinon.

Step 1: Install Jest

First, ensure that Jest is installed in your project. If not, you can install it using npm or yarn:

bash
npm install --save-dev jest @types/jest ts-jest

Step 2: Configure Jest

In the root directory of your project, create or update the jest.config.js file to support TypeScript and other options:

javascript
module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node',
};

Step 3: Create a Mock of EntityManager

You can create a mock of EntityManager in your test files or in dedicated mock files. Here's a simple example:

typescript
// src/__mocks__/EntityManager.ts

export const mockFind = jest.fn();
export const mockSave = jest.fn();
export const EntityManagerMock = jest.fn().mockImplementation(() => ({
  find: mockFind,
  save: mockSave,
}));

// Usage
import { EntityManagerMock, mockFind, mockSave } from './__mocks__/EntityManager';

describe('Test with mocked EntityManager', () => {
  beforeEach(() => {
    mockFind.mockClear();
    mockSave.mockClear();
  });

  test('should use find method of EntityManager', async () => {
    const entityManager = new EntityManagerMock();
    entityManager.find();
    expect(mockFind).toHaveBeenCalled();
  });

  test('should use save method of EntityManager', async () => {
    const entityManager = new EntityManagerMock();
    const entity = { id: 1, name: 'Test' };
    entityManager.save(entity);
    expect(mockSave).toHaveBeenCalledWith(entity);
  });
});

In this example, we use jest.fn() to create mock implementations for the find and save methods, and we integrate these mocks within the EntityManagerMock constructor. This allows you to verify in your tests that these methods are called with the correct parameters.

Step 4: Use the Mock in Tests

As shown in the code above, you can import this mocked EntityManager in your unit tests and use it while leveraging Jest's expect() function to confirm method calls.

This approach enables efficient testing of database-related code without connecting to a real database, thus accelerating tests and minimizing external dependencies.