2. Define schemas + DTOs

*.schemas.ts is the heart of every feature. It holds the zod schemas, the DTO classes (via nestjs-zod's createZodDto), and the extractor function.

// backend/src/tasks/tasks.schemas.ts
import { createZodDto } from 'nestjs-zod';
import { z } from 'zod';

import { TaskRow } from 'src/db/types';

// ========================================
// Response schemas
// ========================================

export const TaskSchema = z.object({
  id: z.string(),
  title: z.string(),
  completed: z.boolean(),
  userId: z.string(),
  createdAt: z.string(),
});

export class TaskDto extends createZodDto(TaskSchema) {}

// List responses are { items, total }, never a bare array.
export const TaskListSchema = z.object({
  items: z.array(TaskSchema),
  total: z.number(),
});

export class TaskListDto extends createZodDto(TaskListSchema) {}

// Delete responses reuse the shared DeleteResultDto from
// src/common/common.schemas. Don't redeclare it per module.

// ========================================
// Request schemas
// ========================================

export const CreateTaskSchema = z.object({
  title: z.string().min(1).max(200),
});
export class CreateTaskDto extends createZodDto(CreateTaskSchema) {}

export const UpdateTaskSchema = z
  .object({
    title: z.string().min(1).max(200).optional(),
    completed: z.boolean().optional(),
  })
  .strict();
export class UpdateTaskDto extends createZodDto(UpdateTaskSchema) {}

// ========================================
// DTO Extractor Functions
// ========================================

export const extractTaskDtoFromRow = (row: TaskRow): TaskDto => ({
  id: row.id,
  title: row.title,
  completed: row.completed,
  userId: row.user_id,
  createdAt: row.created_at.toISOString(),
});

The global ZodValidationPipe validates every @Body() input: TaskDto against the underlying schema, so bad input becomes a 400 with field-level errors without any work in the controller.