**Course Title:** Mastering NestJS: Building Scalable Server-Side Applications **Section Title:** Working with Databases: TypeORM and Data Transfer Objects (DTOs) **Topic:** Handling data with DTOs for validation and transformation **Overview** In the previous topics, we've covered the basics of working with databases using TypeORM in NestJS. We've learned how to create database entities, migrations, and integrate TypeORM with NestJS. In this topic, we'll dive deeper into handling data with Data Transfer Objects (DTOs) for validation and transformation. **What are Data Transfer Objects (DTOs)?** DTOs are classes that represent the data being transferred between the client and server. They are used to transform the data from one format to another, making it easier to work with. DTOs are typically used for validation, serialization, and deserialization of data. **Why use DTOs?** DTOs provide several benefits, including: * **Validation**: DTOs can be used to validate the incoming data, ensuring that it meets the required format and structure. * **Serialization**: DTOs can be used to serialize the data, making it easier to transfer between the client and server. * **Deserialization**: DTOs can be used to deserialize the data, making it easier to work with in the server-side application. * **Decoupling**: DTOs can be used to decouple the client and server, making it easier to change or replace either component without affecting the other. **Creating DTOs in NestJS** To create a DTO in NestJS, you can use the `@nestjs/swagger` package. Here's an example of how to create a DTO using Swagger: ```typescript import { ApiProperty } from '@nestjs/swagger'; export class CreateUserDto { @ApiProperty() name: string; @ApiProperty() email: string; @ApiProperty() password: string; } ``` In this example, we've created a `CreateUserDto` class with three properties: `name`, `email`, and `password`. We've used the `@ApiProperty` decorator to specify the name and description of each property. **Using DTOs for Validation** To use a DTO for validation, you can create a validation pipe in NestJS. Here's an example of how to create a validation pipe that validates the `CreateUserDto` class: ```typescript import { PipeTransform, Injectable } from '@nestjs/common'; import { CreateUserDto } from './create-user.dto'; @Injectable() export class CreateUserValidationPipe implements PipeTransform { transform(value: CreateUserDto): CreateUserDto { if (!value.name || !value.email || !value.password) { throw new Error('Invalid request'); } return value; } } ``` In this example, we've created a `CreateUserValidationPipe` class that implements the `PipeTransform` interface. The `transform` method checks if the `name`, `email`, and `password` properties are present in the `CreateUserDto` object. If any of these properties are missing, it throws an error. **Using DTOs for Serialization and Deserialization** To use a DTO for serialization and deserialization, you can use the `@nestjs/swagger` package. Here's an example of how to serialize and deserialize a `CreateUserDto` object: ```typescript import { Controller, Post, Body, HttpException, HttpStatus } from '@nestjs/common'; import { CreateUserDto } from './create-user.dto'; @Controller('users') export class UsersController { @Post() async createUser(@Body() createUserDto: CreateUserDto): Promise<CreateUserDto> { // Serialize the CreateUserDto object const serializedDto = new CreateUserDto(); serializedDto.name = createUserDto.name; serializedDto.email = createUserDto.email; serializedDto.password = createUserDto.password; // Deserialize the serializedDto object const deserializedDto = new CreateUserDto(); deserializedDto.name = serializedDto.name; deserializedDto.email = serializedDto.email; deserializedDto.password = serializedDto.password; return deserializedDto; } } ``` In this example, we've created a `UsersController` class with a `createUser` method that takes a `CreateUserDto` object as input. The method serializes the `CreateUserDto` object using the `new CreateUserDto()` constructor, and then deserializes the serialized object using the same constructor. **Conclusion** In this topic, we've learned how to use Data Transfer Objects (DTOs) for validation, serialization, and deserialization in NestJS. We've created a `CreateUserDto` class with properties for `name`, `email`, and `password`, and used it to validate, serialize, and deserialize data. We've also created a validation pipe to validate the `CreateUserDto` class, and used it to serialize and deserialize data. **Exercise** Create a `UpdateUserDto` class with properties for `id`, `name`, `email`, and `password`. Use it to update a user in the database. **Solution** ```typescript import { ApiProperty } from '@nestjs/swagger'; export class UpdateUserDto { @ApiProperty() id: number; @ApiProperty() name: string; @ApiProperty() email: string; @ApiProperty() password: string; } ``` ```typescript import { PipeTransform, Injectable } from '@nestjs/common'; import { UpdateUserDto } from './update-user.dto'; @Injectable() export class UpdateUserValidationPipe implements PipeTransform { transform(value: UpdateUserDto): UpdateUserDto { if (!value.id || !value.name || !value.email || !value.password) { throw new Error('Invalid request'); } return value; } } ``` ```typescript import { Controller, Patch, Body, HttpException, HttpStatus } from '@nestjs/common'; import { UpdateUserDto } from './update-user.dto'; @Controller('users') export class UsersController { @Patch(':id') async updateUser(@Param('id') id: number, @Body() updateUserDto: UpdateUserDto): Promise<UpdateUserDto> { // Serialize the UpdateUserDto object const serializedDto = new UpdateUserDto(); serializedDto.id = id; serializedDto.name = updateUserDto.name; serializedDto.email = updateUserDto.email; serializedDto.password = updateUserDto.password; // Deserialize the serializedDto object const deserializedDto = new UpdateUserDto(); deserializedDto.id = serializedDto.id; deserializedDto.name = serializedDto.name; deserializedDto.email = serializedDto.email; deserializedDto.password = serializedDto.password; return deserializedDto; } } ``` **Next Topic** In the next topic, we'll learn how to perform CRUD operations using repositories in NestJS.