Go Gin : Creating generic custom validators

Question

I am using go Gin to create APIs in my project. I have requirement to create custom validators so I created like:

var valueone validator.Func = func(fl validator.FieldLevel) bool {
    value, ok := fl.Field()
    if ok {
        if value != "one" {
                  return true
                }
    }
    return true
}

var valuetwo validator.Func = func(fl validator.FieldLevel) bool {
    value, ok := fl.Field()
    if ok {
        if value != "two" {
                  return true
                }
    }
    return true
}
....

Instead of creating almost same custom validator multiple times, is there a way to create a single validator which is more generic and can be used for both cases, something like:

var value validator.Func = func(fl validator.FieldLevel, param) bool {
    value, ok := fl.Field()
    if ok {
        if value != param {
                  return true
                }
    }
    return true
}

I am not able to find a way to pass parameter to custom validator in gin or to make these generic validators through any other possible way. I have requirement to create like thousands of almost similar validator and I don't want to create custom validator for each one of them.

Answer 1

You can't change the function structure, as this is how it defined within the package.

// Func accepts a FieldLevel interface for all validation needs. The return
// value should be true when validation succeeds.

type Func func(fl FieldLevel) bool

instead, we could try custom validation tag with a parameter

see the sample below

package main

import (
    "github.com/go-playground/validator"
)

type Data struct {
    One string `json:"one" validate:"custom_validation=one"`
    Two string `json:"two" validate:"custom_validation=two"`
}

var validate *validator.Validate

func main() {
    validate = validator.New()

    err := validate.RegisterValidation("custom_validation", func(fl validator.FieldLevel) bool {
        value := fl.Field()
        param := fl.Param()

        return value.String() == param
    })

    if err != nil {
        panic(err)
    }

    // this will succeed
    {
        data := &Data{
            One: "one",
            Two: "two",
        }

        err = validate.Struct(data)
        if err != nil {
            panic(err)
        }
    }

    // it will fail here
    {
        data := &Data{
            Two: "one",
            One: "two",
        }

        err = validate.Struct(data)
        if err != nil {
            panic(err)
        }
    }
}

See more examples at here

Note : Golang doesn't support !==