Path, Query & Unknown-Field Validation UUID parameters, query coercion mental model, strict objects, and layered boundary defense
Phase 12 - DTOs, Zod & Input Validation Volume 04 - Professional Backend
Architecture milestone path/query/body input
↓
schema policy
↓
validated values
Phase 12 introduces an explicit runtime validation boundary before controller and service logic.
Validate every external entry point deliberately Request bodies are only one input source. Path parameters, query strings, headers, and eventually uploaded files all cross trust boundaries. The validation technique depends on the input and phase.
UUID path parameters export const taskIdSchema = z.uuid();
@Param("id", { schema: taskIdSchema }) id: stringThe same schema is reused by GET, PATCH, and DELETE, making the route contract consistent.
Query parameters arrive as strings We intentionally postpone the full Task filtering API until Phase 15, but the mental model starts here. A URL such as ?page=2 still enters as text. If the handler wants a number, the schema must coerce or parse it intentionally.
const pageSchema = z.coerce.number().int().positive();// "2" -> 2 // "banana" -> validation failure
Coercion is convenient, but do not coerce every malformed input into something valid. Decide which representations your API deliberately accepts.
Unknown fields: reject, strip, or preserve? Policy Behavior When it can make sense
TaskFlow Phase 12 chooses reject for write DTOs using z.strictObject. This is a contract choice, not a universal law.
Prototype pollution and framework boundaries NestJS itself includes protections around request transformation and dangerous prototype keys. Validation still matters because security hardening and semantic correctness are different concerns: “this object cannot poison a prototype” does not mean “this is a valid Task request.”
Figure 1 - Layered boundary defense HTTP framework parsing
↓
framework safety checks
↓
schema validation
↓
business authorization/rules later
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database constraints
Security and correctness come from layered controls, not one magical validator.
Path Query and Unknown Field 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 “Path Query and Unknown Field 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: Reusable create/update/query schemas integrated into NestJS request handling with useful validation errors.. 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 “Path Query and Unknown Field Validation” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
List the exact fields accepted by create-task and reject unknown/invalid shapes deliberately.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Putting database existence checks into pure syntax schemas
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.