Use enum validation in golang with gin with custom error messages

validate_enum.go

// user model
type User struct {
	gorm.Model
	Name string `json:"name" binding:"required" gorm:"not null:true"`
	Phone string `json:"phone" binding:"required"`
	Email string `json:"email" binding:"required,email" gorm:"not null:true"`
	Password string `json:"password" binding:"required,min=8" gorm:"not null:true"`
	Gender string `json:"gender" binding:"Enum=male_female" gorm:"type:gender;not null:true;default:male"` // add binding Enum=male_female
}


// create custom validation
func Enum(
	fl validator.FieldLevel,
) bool {
	enumString := fl.Param() // get string male_female
	value := fl.Field().String() // the actual field
	enumSlice := strings.Split(enumString, "_") // convert to slice
	for _, v := range enumSlice {
		if value == v {
			return true
		}
	}
	return false
}


// register the validation in main function
if v, ok := binding.Validator.Engine().(*validator.Validate); ok {
		_ = v.RegisterValidation("Enum", Enum)
}

// you can now easily validate
var user User
if err := ctx.ShouldBind(&user); err != nil {
  ctx.JSON(http.StatusBadRequest, gin.H{"error": ValidateErrors(err)}) // ValidateErrors is custom error handler to display custom message
  return
}

// custom validation error messages
func ValidateErrors(requestError error) string {
	return validate(requestError.(validator.ValidationErrors))
}

func validate(errors validator.ValidationErrors) string {
	resultErrors := ""
	for _, err := range errors {
		switch err.Tag() {
		case "required":
			resultErrors += err.Field() + " is required\n "
		case "email":
			resultErrors += err.Field() + " must me valid email\n "
		case "min":
			resultErrors += err.Field() + " must be " + err.Param() + " length at least\n"
		case "Enum":
			replacer := *strings.NewReplacer("_", ",")
			resultErrors += err.Field() + " must be one of " + replacer.Replace(err.Param())


		default:
			resultErrors += "error in filed " + err.Tag()
		}
	}
	return resultErrors
}

// example of error returns {
//  "error": "Name is required\n Phone is required\n Email must me valid email\n Password must be 8 length at least\nGender must be one of male,female"
// }