go 57 lines · 8 steps

Custom validators and binding in Gin

Register a reusable phone-number rule with Gin's validator and turn binding failures into structured field-level error responses.

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Gin Intermediate validation struct-tags error-handling regex sync-once
1package handlers
2 
3import (
4 "net/http"
5 "regexp"
6 "sync"
7 
8 "github.com/gin-gonic/gin"
9 "github.com/gin-gonic/gin/binding"
10 "github.com/go-playground/validator/v10"
11)
12 
13var (
14 phoneRegex = regexp.MustCompile(`^\+?[1-9]\d{1,14}$`)
15 registerPhone sync.Once
16)
17 
18func RegisterValidators() {
19 registerPhone.Do(func() {
20 if v, ok := binding.Validator.Engine().(*validator.Validate); ok {
21 v.RegisterValidation("e164", func(fl validator.FieldLevel) bool {
22 return phoneRegex.MatchString(fl.Field().String())
23 })
24 }
25 })
26}
27 
28type CreateContactRequest struct {
29 Name string `json:"name" binding:"required,min=2"`
30 Phone string `json:"phone" binding:"required,e164"`
31 Email string `json:"email" binding:"required,email"`
32}
33 
34func CreateContact(c *gin.Context) {
35 var req CreateContactRequest
36 if err := c.ShouldBindJSON(&req); err != nil {
37 var verrs validator.ValidationErrors
38 if errors.As(err, &verrs) {
39 fields := make(map[string]string, len(verrs))
40 for _, fe := range verrs {
41 fields[fe.Field()] = fe.Tag()
42 }
43 c.JSON(http.StatusUnprocessableEntity, gin.H{"errors": fields})
44 return
45 }
46 c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
47 return
48 }
49 
50 contact, err := contacts.Create(c.Request.Context(), req.Name, req.Phone, req.Email)
51 if err != nil {
52 c.JSON(http.StatusInternalServerError, gin.H{"error": "could not create contact"})
53 return
54 }
55 
56 c.JSON(http.StatusCreated, contact)
57}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Registering a custom validation tag once lets every request struct reuse it declaratively via binding tags.
  2. 2Type-asserting binding errors to validator.ValidationErrors lets you report exactly which fields failed and why.
  3. 3Distinguishing validation failures from malformed input maps cleanly onto different HTTP status codes.

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