go 42 lines · 5 steps

Custom validators in Gin

Register reusable struct-tag validators on Gin's binding engine, safely and exactly once.

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Gin Intermediate validation sync.once regexp type-assertion struct-tags
1package validators
2 
3import (
4 "regexp"
5 "sync"
6 "time"
7 
8 "github.com/gin-gonic/gin/binding"
9 "github.com/go-playground/validator/v10"
10)
11 
12var (
13 slugPattern = regexp.MustCompile(`^[a-z0-9]+(?:-[a-z0-9]+)*$`)
14 once sync.Once
15)
16 
17func Register() {
18 once.Do(func() {
19 v, ok := binding.Validator.Engine().(*validator.Validate)
20 if !ok {
21 return
22 }
23 v.RegisterValidation("slug", validateSlug)
24 v.RegisterValidation("notpast", validateNotPast)
25 })
26}
27 
28func validateSlug(fl validator.FieldLevel) bool {
29 s := fl.Field().String()
30 if s == "" {
31 return true
32 }
33 return slugPattern.MatchString(s)
34}
35 
36func validateNotPast(fl validator.FieldLevel) bool {
37 ts, ok := fl.Field().Interface().(time.Time)
38 if !ok {
39 return false
40 }
41 return !ts.Before(time.Now())
42}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Registering validators through sync.Once keeps setup idempotent even if called from multiple init paths.
  2. 2Custom validators are plain functions that read fl.Field() and return a bool verdict.
  3. 3Treating an empty string as valid lets you compose a format rule with a separate required rule.

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