How to cover TypeORM @Column decorator with Jest unit testing?

When using Jest for unit testing, we typically focus on the logical aspects of the code to ensure it runs as expected. For the @Column decorator in TypeORM, as it primarily defines how class properties map to database columns, it is generally unnecessary to directly test the decorator itself. Instead, we can indirectly verify that our decorator configuration is correct by testing the behavior of entities that utilize the @Column decorator.

1. Setting Up the Environment

First, ensure Jest and TypeORM are installed in your project. You can install them with the following commands (if not already installed):

bash
npm install --save-dev jest
npm install typeorm

2. Creating the Entity Class

Assume we have a simple user entity class that uses the @Column decorator to define properties:

typescript
// user.entity.ts
import { Entity, PrimaryGeneratedColumn, Column } from "typeorm";

@Entity()
export class User {
    @PrimaryGeneratedColumn()
    id: number;

    @Column()
    name: string;

    @Column()
    age: number;
}

3. Writing Test Cases

In the tests, we create an instance and verify that properties are handled correctly. While this does not directly test the @Column decorator, it helps confirm that the entity behaves as expected:

typescript
// user.test.ts
import { User } from "./user.entity";

describe('User entity', () => {
    it('should create a user with properties', () => {
        const user = new User();
        user.name = "Tom";
        user.age = 25;

        expect(user.name).toBe("Tom");
        expect(user.age).toBe(25);
    });
});

4. Running the Tests

After configuring Jest, you can execute the tests using npm test or jest.

Conclusion

Although this test example does not directly validate the @Column decorator, it ensures that instances of the User class using the decorator function as intended. In practice, we typically focus on the overall behavior of entities interacting with the database, which is usually covered in integration tests or end-to-end tests. For unit tests, our primary concern is the correctness of the class logic. To verify database mapping accuracy, configure data mocking or an integration test environment for comprehensive validation.

Unit testing properties decorated with TypeORM @Column using Jest is primarily achieved by testing instance methods associated with those properties. Specifically for the @Column decorator, we typically do not directly test it as it is part of TypeORM; instead, we indirectly test it by examining how the property functions within entity behavior.

Here's how I organize unit testing for entity classes using the @Column decorator:

1. Entity Class Setup: First, ensure you have an entity class using the @Column decorator. For example:

typescript
import { Entity, PrimaryGeneratedColumn, Column } from 'typeorm';

@Entity()
class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;

  @Column()
  email: string;

  // Some methods may use these properties, for example:
  getFullName() {
    return `${this.name}`;
  }

  // You can also include methods for modifying or validating properties
  updateEmail(newEmail: string) {
    // There might be additional validation logic
    this.email = newEmail;
  }
}

1. Testing Instance Methods: Next, you can write Jest tests to verify methods of the User entity. Although these methods may involve properties decorated with @Column, we focus on the behavior of the methods rather than the decorator itself. For example:

typescript
describe('User', () => {
  it('should return full name', () => {
    const user = new User();
    user.name = 'John Doe';

    expect(user.getFullName()).toBe('John Doe');
  });

  it('should update email', () => {
    const user = new User();
    user.email = 'old@example.com';
    user.updateEmail('new@example.com');

    expect(user.email).toBe('new@example.com');
  });

  // If there is validation logic, it should also be tested
});

1. Integration Testing: If you want to further test the functionality of the TypeORM library itself, such as ensuring the @Column decorator correctly maps properties to database columns, you would perform integration testing rather than unit testing. In integration testing, you interact directly with the database to verify the mapping between entities and the database.

2. Test Coverage: Ensure test cases cover all scenarios related to @Column properties, including validation logic, exception handling, and boundary conditions. Using Jest's coverage report feature helps identify untested code sections.

Although we do not directly test the @Column decorator, by using the above methods, we can ensure that business logic involving these properties is thoroughly tested. This approach guarantees code robustness and ensures that interactions with the database layer proceed as expected.