**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** Working with Databases: TypeORM and Data Transfer Objects (DTOs) **Topic:** Build a data model for a blog application, implementing CRUD operations using TypeORM and DTOs.(Lab topic) **Objective:** By the end of this topic, you will be able to design and implement a data model for a blog application using TypeORM and Data Transfer Objects (DTOs), and perform CRUD (Create, Read, Update, Delete) operations using repositories. **Prerequisites:** * Familiarity with NestJS and its ecosystem * Understanding of TypeORM and its features * Knowledge of Data Transfer Objects (DTOs) and their purpose **Step 1: Designing the Data Model** To start, let's design the data model for our blog application. We will use TypeORM to define the entities and their relationships. * **Entity 1: Post** + `id` (primary key) + `title` + `content` + `authorId` (foreign key referencing the `User` entity) * **Entity 2: User** + `id` (primary key) + `name` + `email` We will use the `@Entity` decorator to define the entities and their properties. ```typescript import { Entity, Column, PrimaryGeneratedColumn, ManyToOne } from 'typeorm'; @Entity() export class Post { @PrimaryGeneratedColumn() id: number; @Column() title: string; @Column() content: string; @ManyToOne(() => User, (user) => user.id) author: User; } @Entity() export class User { @PrimaryGeneratedColumn() id: number; @Column() name: string; @Column() email: string; } ``` **Step 2: Creating the Data Model using TypeORM** Next, we will use TypeORM to create the database schema based on our entities. ```typescript import { createConnection } from 'typeorm'; async function main() { const connection = await createConnection({ type: 'sqlite', database: 'blog.db', entities: [Post, User], synchronize: true, }); console.log('Database schema created successfully!'); } main(); ``` **Step 3: Creating DTOs** Now, let's create DTOs to handle data transfer between the client and the server. ```typescript import { Exclude, Expose } from 'class-transformer'; export class PostDTO { @Expose() id: number; @Expose() title: string; @Expose() content: string; @Expose() author: UserDTO; } export class UserDTO { @Expose() id: number; @Expose() name: string; @Expose() email: string; } ``` **Step 4: Implementing CRUD Operations** Finally, let's implement CRUD operations using repositories. ```typescript import { Repository } from 'typeorm'; import { Post } from './post.entity'; import { PostDTO } from './post.dto'; export class PostRepository { private postRepository: Repository<Post>; constructor(postRepository: Repository<Post>) { this.postRepository = postRepository; } async create(postDTO: PostDTO): Promise<Post> { const post = new Post(); post.title = postDTO.title; post.content = postDTO.content; post.author = postDTO.author; return this.postRepository.save(post); } async findAll(): Promise<Post[]> { return this.postRepository.find(); } async findOne(id: number): Promise<Post> { return this.postRepository.findOne(id); } async update(id: number, postDTO: PostDTO): Promise<Post> { const post = await this.findOne(id); post.title = postDTO.title; post.content = postDTO.content; post.author = postDTO.author; return this.postRepository.save(post); } async delete(id: number): Promise<void> { await this.postRepository.delete(id); } } ``` **Conclusion:** In this topic, we designed and implemented a data model for a blog application using TypeORM and Data Transfer Objects (DTOs), and performed CRUD operations using repositories. We also created DTOs to handle data transfer between the client and the server. **Best Practices:** * Use TypeORM to define entities and their relationships. * Use DTOs to handle data transfer between the client and the server. * Implement CRUD operations using repositories. **Additional Resources:** * TypeORM documentation: <https://typeorm.io/> * Class-transformer documentation: <https://github.com/typestack/class-transformer> **Leave a comment or ask for help if you have any questions or need further clarification on any of the concepts covered in this topic.**