Mastering TypeORM Transaction Mocking: A Step-by-Step Guide to Behavior Assertion in Jest

Are you tired of struggling with TypeORM’s Entity Manager transaction mocking in your Jest unit tests? Do you want to ensure your database interactions are thoroughly tested without affecting your production database? Look no further! In this comprehensive article, we’ll dive into the world of TypeORM transaction mocking, providing you with a clear, step-by-step guide on how to assert behavior when unit testing with Jest.

Why Mock TypeORM’s Entity Manager Transaction?

Before we dive into the “how,” let’s discuss the “why.” Mocking TypeORM’s Entity Manager transaction is crucial for several reasons:

• Isolation**: By mocking the transaction, you can isolate your unit tests from the production database, ensuring that your tests don’t interfere with your live data.
• Speed**: Mocking transactions reduces the time it takes to run your tests, as you’re not waiting for database queries to complete.
• Reliability**: Mocking transactions eliminates the risk of test failures due to database connections or query issues.
• Flexibility**: You can test different scenarios and edge cases without worrying about affecting your production database.

Prerequisites

Before we begin, make sure you have the following installed:

• TypeORM (^0.2.25)
• Jest (^26.6.3)
• a basic understanding of TypeORM and Jest

Step 1: Create a Mock Entity Manager

To create a mock Entity Manager, we’ll use Jest’s built-in mocking features. In your test file, add the following code:

import { EntityManager } from 'typeorm';
import { createMockEntityManager } from '../mocks/entity-manager.mock';

jest.mock('typeorm', () => ({
  ...jest.requireActual('typeorm'),
  getEntityManager: () => createMockEntityManager(),
}));

const createMockEntityManager = (): EntityManager => {
  return {
    transaction: jest.fn(() => Promise.resolve()),
    query: jest.fn(() => Promise.resolve()),
    findOne: jest.fn(() => Promise.resolve()),
    // Add more methods as needed
  };
};

In this example, we’re creating a mock `EntityManager` object with a `transaction` method that returns a resolved promise. We’re also defining other methods like `query` and `findOne`, which will be useful for our test scenarios. You can add more methods as needed, depending on your use case.

Step 2: Implement Transaction Mocking

Now that we have our mock Entity Manager, let’s implement transaction mocking. Create a new file, `transaction.mock.ts`, with the following code:

import { EntityManager } from 'typeorm';

interface TransactionMock {
  execute: jest.Mock Promise]>;
  commit: jest.Mock;
  rollback: jest.Mock;
}

const transactionMock: TransactionMock = {
  execute: jest.fn(() => Promise.resolve()),
  commit: jest.fn(),
  rollback: jest.fn(),
};

jest.mock('typeorm', () => ({
  ...jest.requireActual('typeorm'),
  EntityManager: class EntityManager {
    transaction: () => transactionMock;
  },
}));

In this example, we’re creating a `TransactionMock` interface with three methods: `execute`, `commit`, and `rollback`. We’re then defining these methods as Jest mocks, allowing us to control their behavior in our tests.

Step 3: Write Your Unit Tests

Now that we have our mock Entity Manager and transaction mocking in place, let’s write some unit tests. Create a new file, `my.service.spec.ts`, with the following code:

import { MyService } from './my.service';
import { createMockEntityManager } from '../mocks/entity-manager.mock';
import { transactionMock } from '../mocks/transaction.mock';

describe('MyService', () => {
  let myService: MyService;
  let entityManager: EntityManager;

  beforeEach(() => {
    entityManager = createMockEntityManager();
    myService = new MyService(entityManager);
  });

  it('should execute a transaction and commit', async () => {
    const executeCallback = jest.fn(() => Promise.resolve());
    transactionMock.execute.mockImplementation(executeCallback);

    await myService.doSomething();

    expect(transactionMock.execute).toHaveBeenCalledTimes(1);
    expect(executeCallback).toHaveBeenCalledTimes(1);
    expect(transactionMock.commit).toHaveBeenCalledTimes(1);
  });

  it('should roll back a transaction on error', async () => {
    const executeCallback = jest.fn(() => Promise.reject(new Error('Something went wrong')));
    transactionMock.execute.mockImplementation(executeCallback);

    await expect(myService.doSomething()).rejects.toThrowError('Something went wrong');

    expect(transactionMock.execute).toHaveBeenCalledTimes(1);
    expect(executeCallback).toHaveBeenCalledTimes(1);
    expect(transactionMock.rollback).toHaveBeenCalledTimes(1);
  });
});

In this example, we’re testing a `MyService` class that uses the `EntityManager` to perform database operations. We’re using our mock Entity Manager and transaction mocking to control the behavior of the `execute`, `commit`, and `rollback` methods.

Step 4: Verify Mock Behavior

In our tests, we’re verifying that the mock `execute` method is called with the correct callback function, and that the `commit` or `rollback` methods are called depending on the test scenario. We’re also using Jest’s expect syntax to assert that the mock methods are called the correct number of times.

Conclusion

By following these steps, you’ve successfully mocked TypeORM’s Entity Manager transaction for behavior assertion in Jest. You can now write robust unit tests that ensure your database interactions are correct, without affecting your production database.

Remember to adjust the mocking strategy to fit your specific use case. Happy testing!

Mocking Strategy	Pros	Cons
Manual Mocking	Full control over mock behavior	Time-consuming and prone to errors
Jest’s Auto-Mocking	Easy to set up, automatic mocking	Limited control over mock behavior
Hybrid Approach	Combines manual and auto-mocking for flexibility	Requires more setup and configuration

Choose the mocking strategy that best fits your needs, and don’t hesitate to experiment with different approaches as you continue to master TypeORM transaction mocking.

Frequently Asked Question

Get ready to master the art of unit testing with TypeORM’s Entity Manager Transaction!