DTOs & Validation

Why this matters

DTOs (Data Transfer Objects) describe what data is allowed in an API request. We’ll use Zod, a TypeScript-first schema library, together with nestjs-zod so that validation happens automatically.

What is Zod? Zod lets you declare data shapes using JavaScript/TypeScript. It produces readable error messages and can be reused for both runtime validation and type inference. See the official docs for deeper study.

How this fits into the app

Requests → DTOs → Service: When an HTTP request hits a controller, Nest converts the JSON body into a DTO instance. If the DTO passes validation, the data is safe to pass to your service.
Zod schemas drive validation: By declaring the shape of CreateTask and UpdateTask, we can guarantee required fields (like title) are present and properly formatted before touching the database.
Shared types: Because Zod schemas are written in TypeScript, they also describe the TypeScript types you can reuse across services or even a front-end later.

Beginner tip

Focus on the idea that DTOs are the gatekeepers of your API. They define what “good input” looks like. Zod just gives us a nice, readable way to write those rules.

1. Create DTO files

Make a new folder src/tasks/dto/ and add two files.

mkdir -p src/tasks/dto

src/tasks/dto/create-task.dto.ts

import { createZodDto } from 'nestjs-zod';
import { z } from 'zod';

export const CreateTaskSchema = z.object({
  title: z
    .string()
    .min(1, 'Title is required')
    .max(80, 'Keep the title under 80 characters'),
  description: z
    .string()
    .max(200, 'Description should be short')
    .optional(),
});

export class CreateTaskDto extends createZodDto(CreateTaskSchema) {}

src/tasks/dto/update-task.dto.ts

import { createZodDto } from 'nestjs-zod';
import { z } from 'zod';
import { CreateTaskSchema } from './create-task.dto';

export const UpdateTaskSchema = CreateTaskSchema.extend({
  done: z.boolean().optional(),
}).partial();

export class UpdateTaskDto extends createZodDto(UpdateTaskSchema) {}

createZodDto turns a Zod schema into a Nest-compatible DTO class.
partial() makes every field optional for updates.

2. Enable validation globally

Update src/main.ts to use the ZodValidationPipe provided by nestjs-zod.

src/main.ts

import { NestFactory } from '@nestjs/core';
import { ZodValidationPipe } from 'nestjs-zod'; 
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalPipes(new ZodValidationPipe()); 
  await app.listen(process.env.PORT ?? 3000);
}
bootstrap();

Now every incoming request is checked against the Zod schemas before reaching your controllers.

3. Try it out (optional)

Start the server (npm run start:dev) and send a POST request without a title. You should receive a 400 Bad Request with a descriptive Zod validation error.

Commit DTO and validation updates

Capture the DTO files and validation wiring before moving on:

git add src/tasks/dto/ src/main.ts
git commit -m "feat: add task dtos and validation"

Push your branch and open a pull request

Stay on your feature/data-modeling branch and push it to GitHub:

git push -u origin feature/data-modeling

Then open a pull request targeting main. Add a short summary (what changed, any screenshots of DTO usage) and request a teammate review. When reviewers approve, choose Squash and merge so the entire branch becomes a single commit on main (for example, feat: add task data modeling).

Once merged, pull the latest main locally before starting the next chapter.

Next, move to Implement CRUD Service to add database operations.