Query DTOs & Zod Validation Normalize strings, coerce numbers, apply defaults, cap page size, and reject unsupported parameters
Phase 15 - Filtering, Search, Sorting & Pagination Volume 04 - Professional Backend
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Phase 15 turns an unbounded collection endpoint into a validated, queryable, stable, paginated API contract.
Validate query strings at the HTTP boundary Query strings arrive as strings. page=2 is initially the string "2", not the number 2. Phase 15 uses Zod coercion for numeric pagination fields and strict allow-lists for enums and sort options.
import { z } from "zod";
import {
TaskPriority,
TaskStatus,
} from "../generated/prisma/client";export const taskSortFieldSchema = z.enum([ "createdAt", "updatedAt", "dueAt", "title", ]);
export const sortOrderSchema = z.enum(["asc", "desc"]);export const taskListQuerySchema = z.strictObject({ projectId: z.uuid().optional(), assigneeId: z.uuid().optional(), status: z.enum(TaskStatus).optional(), priority: z.enum(TaskPriority).optional(), search: z.string().trim().min(1).max(100).optional(), sort: taskSortFieldSchema.default("createdAt"), order: sortOrderSchema.default("desc"), page: z.coerce.number().int().min(1).default(1), pageSize: z.coerce.number().int().min(1).max(100).default(20), });
export type TaskListQueryDto = z.infer<typeof taskListQuerySchema>;
export type TaskSortField = z.infer<typeof taskSortFieldSchema>;
export type SortOrder = z.infer<typeof sortOrderSchema>;Why defaults belong in the boundary schema Defaults such as page=1, pageSize=20, sort=createdAt, and order=desc are part of the API contract. Applying them once at validation means TasksService receives a normalized query instead of repeatedly handling undefined values.
/tasks page=1, pageSize=20, createdAt desc
?page=0 400 - page must be >= 1
?pageSize=1000 400 - server cap is 100
?sort=dropTable 400 - field is not allow-listed
?status=almost_done 400 - invalid enum value
?search= 400 after trim/min length
Integrate with NestJS 12 @Get() findAll( @Query(new StandardSchemaValidationPipe(taskListQuerySchema)) query: TaskListQueryDto, ) { return this.tasksService.findAll(query); }
Do not pass raw query strings into Prisma. Normalize and constrain them first. This is both safer and easier to reason about.
Why pageSize has a hard maximum Without a server-controlled cap, a client can request pageSize=1000000 and effectively bypass pagination. The product may later choose 50 or 200, but the backend must own the upper bound.
Query DTOs and Zod Validation matters because it changes a real boundary in TaskFlow. The goal is to understand the inputs, owner, observable output, and failure modes before adding more abstraction.
Restate the goal of “Query DTOs and Zod Validation” as one observable behavior. Write the success signal before touching code.
Trace the boundary map: untrusted input → runtime schema → validated value → business rule → safe operation. Mark the one boundary this lesson changes.
Inspect the current repository state and record the exact files, route, table, or runtime involved. Avoid creating an abstraction before you can name the existing owner.
Implement the smallest vertical slice. Add one coherent change, run the narrowest verification command, and read the output before continuing.
Connect the slice to TaskFlow’s phase artifact: Task list API with filter/search/sort/page contract plus metadata and indexes justified by measured query patterns.. Verify behavior from the outside as a user or API client would.
Create one controlled failure that is safe to reproduce. Predict the symptom first, capture evidence, locate the earliest broken boundary, then fix only that cause.
Close the reference and rebuild or explain the core change from memory. Record what you would monitor or test before trusting the same change in production.
const CreateTaskSchema = z.object({
title: z.string().trim().min(1).max(160),
priority: z.enum(["LOW", "MEDIUM", "HIGH"]),
});
const input = CreateTaskSchema.parse(requestBody);Do not copy this as a finished answer. Predict what each boundary does, type the smallest useful slice, run it, then compare the result with your prediction.
Return to the nearest observable boundary: input, network, process log, database, or rendered UI. Compare actual evidence with the predicted path.
Change one variable at a time and reproduce the original failure after the fix; otherwise you have correlation, not diagnosis.
Without copying the final source, create a second small scenario that applies “Query DTOs and Zod Validation” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Whitelist sortable fields and directions instead of interpolating arbitrary user input.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Nondeterministic order causing duplicates/missing rows
Predict the symptom before causing the failure. Then locate the earliest boundary where reality diverges from expectation.Why does this responsibility live here, what input does it trust, what output proves success, and what would go wrong if another layer owned it?
If your explanation is vague, repeat the boundary trace before moving on.Close the reference. Reproduce the core idea from memory or explain the exact request/data flow aloud. Then check your diff or notes for accidental shortcuts that bypass validation, authorization, error handling, or verification earned in earlier phases.