Create Task Validation Validate POST bodies before Project lookup or Prisma persistence
Phase 12 - DTOs, Zod & Input Validation Volume 04 - Professional Backend
Architecture milestone POST /tasks
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createTaskSchema
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service Project check
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Phase 12 introduces an explicit runtime validation boundary before controller and service logic.
POST /tasks - validate before creating The create endpoint is an ideal place to see the validation boundary because it accepts multiple fields and then performs a real parent-existence business check.
Figure 1 - Valid create request POST JSON
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createTaskSchema parses
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Input validation is first; business validation and persistence follow.
Valid request { "projectId": "3d9c5d68-4d2f-4db0-a48a-48b5adf0988b", "title": " Validate TaskFlow input ", "description": "Schema-first validation" }
The title received by the service is trimmed because the Zod schema produces the parsed output.
Invalid: blank title { "projectId": "3d9c5d68-4d2f-4db0-a48a-48b5adf0988b", "title": " " }
Figure 2 - Blank-title failure raw body
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trim title
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empty string
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min(1) fails
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The Project lookup never runs because shape validation fails earlier.
Invalid: malformed projectId { "projectId": "project-123", "title": "Should not reach the service" }
This fails UUID syntax validation. That is different from a syntactically valid UUID that simply does not identify an existing Project; the latter reaches TasksService and becomes a Project-not-found business result.
Invalid: unknown property { "projectId": "3d9c5d68-4d2f-4db0-a48a-48b5adf0988b", "title": "Should fail", "admin": true }
Strict request schemas make accidental contract expansion visible. They also stop a client from believing arbitrary extra fields influence server behavior.
PowerShell testing A browser UI is not required to prove the boundary. Keep testing the backend independently.
$body = @{ projectId = "<PROJECT-ID>" title = "Validated from PowerShell" } | ConvertTo-Json
Invoke-RestMethod ` -Method Post ` -Uri "http://localhost:4000/tasks" ` -ContentType "application/json" ` -Body $body
Create Task 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 “Create Task 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 “Create Task Validation” to a different TaskFlow resource or input. Write the acceptance criteria first, implement it, and provide the evidence that each criterion is satisfied.
Validate query parameters such as page size/sort values instead of trusting strings.
Use the smallest change that demonstrates this lesson. Stop after it works and capture evidence.Assuming TypeScript validates runtime JSON
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.