**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** Working with Databases: TypeORM and Data Transfer Objects (DTOs) **Topic:** Performing CRUD operations using repositories **Overview** In this topic, we will explore how to perform Create, Read, Update, and Delete (CRUD) operations using repositories in a NestJS application. We will use TypeORM as our database ORM (Object-Relational Mapping) tool to interact with our database. **What are Repositories?** Repositories are a design pattern that abstracts the data access layer of an application. They provide a layer of indirection between the business logic and the data storage, making it easier to switch between different data storage solutions. **Why Use Repositories?** Using repositories provides several benefits, including: * **Decoupling**: Repositories decouple the business logic from the data storage, making it easier to switch between different data storage solutions. * **Reusability**: Repositories can be reused across multiple controllers and services, reducing code duplication. * **Testability**: Repositories make it easier to test the business logic by providing a mockable interface. **Creating a Repository** To create a repository, we need to create a new class that extends the `Repository` class from TypeORM. We can then use the `@InjectRepository` decorator to inject the repository into our service or controller. ```typescript import { EntityRepository, Repository } from 'typeorm'; import { User } from './user.entity'; @EntityRepository(User) export class UserRepository extends Repository<User> {} ``` **Performing CRUD Operations** Once we have created our repository, we can use it to perform CRUD operations on our data. ### Create To create a new entity, we can use the `save` method of the repository. ```typescript async createUser(user: User): Promise<User> { return this.repository.save(user); } ``` ### Read To read an entity, we can use the `findOne` method of the repository. ```typescript async getUser(id: number): Promise<User | undefined> { return this.repository.findOne(id); } ``` ### Update To update an entity, we can use the `update` method of the repository. ```typescript async updateUser(id: number, user: User): Promise<void> { await this.repository.update(id, user); } ``` ### Delete To delete an entity, we can use the `delete` method of the repository. ```typescript async deleteUser(id: number): Promise<void> { await this.repository.delete(id); } ``` **Best Practices** When using repositories, it's a good practice to: * **Use a consistent naming convention**: Use a consistent naming convention for your repository classes and methods. * **Use type annotations**: Use type annotations to specify the types of the parameters and return values of your repository methods. * **Use async/await**: Use async/await to write asynchronous code that is easier to read and maintain. **Conclusion** In this topic, we have learned how to perform CRUD operations using repositories in a NestJS application. We have also discussed the benefits of using repositories and provided best practices for using them. **What's Next?** In the next topic, we will discuss best practices for error handling in NestJS applications. **Leave a comment or ask for help if you have any questions or need further clarification on any of the concepts discussed in this topic.** **External Resources:** * [TypeORM Documentation](https://typeorm.io/) * [NestJS Documentation](https://docs.nestjs.com/)