JSON schema-ish validation with custom error details

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                  package api

import "strings"

type Issue struct {
  Field   string `json:"field"`
  Message string `json:"message"`
}

type CreateReq struct {
  Name  string `json:"name"`
  Email string `json:"email"`
}

func (r CreateReq) Validate() []Issue {
  issues := make([]Issue, 0)
  if strings.TrimSpace(r.Name) == "" {
    issues = append(issues, Issue{Field: "name", Message: "is required"})
  }
  if !strings.Contains(r.Email, "@") {
    issues = append(issues, Issue{Field: "email", Message: "must be a valid email"})
  }
  return issues
}

I don’t try to re-implement full JSON Schema in Go, but I do like returning validation errors that are easy for clients to render. The pattern is: decode into a struct, validate required fields and invariants, and return a slice of {field, message} issues. This gives you a stable contract: the frontend can highlight fields without parsing English strings. I keep the validation separate from the handler so it’s testable and so the handler remains a thin adapter. For more complex domains I’d pull in a validation library, but the “hand-rolled issues array” scales surprisingly well for typical CRUD APIs. The important detail is consistency: every endpoint should return the same VALIDATION_ERROR code and the same issue shape. That uniformity is what makes client code simple.

Leah Thompson

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JSON schema-ish validation with custom error details

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