user.go

// ...

// Create new validator
validate := utils.Validate(map[string]string{
  "uuid":     user.ID.String(),
  "email":    user.Email,
  "username": user.Username,
})

// Check fields validation
if errValidate := validate.Struct(user); errValidate != nil {
  // Return invalid fields
  c.Status(500).JSON(fiber.Map{"error": true, "msg": utils.ValidateErrors(errValidate)})
  return
}

// ...

validate.go

package utils

import (
	"reflect"
	"regexp"
	"strings"

	"github.com/go-playground/validator/v10"
	"github.com/google/uuid"
)

// Validate create a new validator for expected fields,
// register function to get tag name from `json` tags
// and add validation to expected fields
func Validate(fields map[string]string) *validator.Validate {
	// Create a new validator
	validate := validator.New()

	// Get tag name from `json`
	validate.RegisterTagNameFunc(func(fld reflect.StructField) string {
		name := strings.SplitN(fld.Tag.Get("json"), ",", 2)[0]
		if name == "-" {
			return ""
		}
		return name
	})

	// Add fields validation
	for tag, field := range fields {
		_ = validate.RegisterValidation(tag, func(fl validator.FieldLevel) bool {
			// Check each field
			return ValidateField(tag, field)
		})
	}

	return validate
}

// ValidateField switched field tag for right validation
func ValidateField(tag, field string) bool {
	switch tag {
	case "uuid":
		// Check for valid UUID
		if _, errParseUUID := uuid.Parse(field); errParseUUID != nil {
			return false
		}
	case "email":
		// Check for regexp parrtern, length (varchar(255)) and empty string
		if !regexp.MustCompile("^.+@.+\\..+$").MatchString(field) || len(field) > 254 || field == "" {
			return false
		}
	case "username":
		// Check for length (varchar(13)) and empty string
		if len(field) > 13 || field == "" {
			return false
		}
	}

	return true
}

// ValidateErrors show validation error for each invalid fields
func ValidateErrors(err error) map[string]string {
	errorFields := map[string]string{}
	for _, err := range err.(validator.ValidationErrors) {
		// Make error message
		errorFields[err.Field()] = "field " + err.StructField() + " is not valid"
	}

	return errorFields
}