CUSTOM_RULE.md

Validate with custom rule

You can register custom validation rules. This rule will work for both Validate and ValidateJSON method. You will get all the information you need to validate an input.

package main

import (
	"encoding/json"
	"errors"
	"fmt"
	"net/http"
	"strconv"
	"strings"

	"github.com/thedevsaddam/govalidator"
)

func init() {
	// custom rules to take fixed length word.
	// e.g: max_word:5 will throw error if the field contains more than 5 words
	govalidator.AddCustomRule("max_word", func(field string, rule string, message string, value interface{}) error {
		valSlice := strings.Fields(value.(string))
		l, _ := strconv.Atoi(strings.TrimPrefix(rule, "max_word:")) //handle other error
		if len(valSlice) > l {
			if message != "" {
				return errors.New(message)
			}
			return fmt.Errorf("The %s field must not be greater than %d words", field, l)
		}
		return nil
	})
}

type article struct {
	Title string   `json:"title"`
	Body  string   `json:"body"`
	Tags  []string `json:"tags"`
}

func handler(w http.ResponseWriter, r *http.Request) {
	var article article
	rules := govalidator.MapData{
		"title": []string{"between:10,120"},
		"body":  []string{"max_word:150"}, // using custom rule max_word
		"tags":  []string{"between:3,5"},
	}

	opts := govalidator.Options{
		Request:         r,
		Data:            &article,
		Rules:           rules,
		RequiredDefault: true, //force user to fill all the inputs
	}

	v := govalidator.New(opts)
	e := v.ValidateJSON()
	err := map[string]interface{}{"validationError": e}
	w.Header().Set("Content-type", "applciation/json")
	json.NewEncoder(w).Encode(err)
}

func main() {
	http.HandleFunc("/", handler)
	fmt.Println("Listening on port: 9000")
	http.ListenAndServe(":9000", nil)
}

Resposne

{
    "validationError": {
        "body": [
            "The body field must not be greater than 150 words"
        ],
        "tags": [
            "The tags field must be between 3 and 5"
        ],
        "title": [
            "The title field must be between 10 and 120"
        ]
    }
}