How can I validate a struct datatype with a custom validator?

Question

I am using go-playground/validator/v10 to validate some input and have some trouble with custom validation tags and functions. The problem is that the function is not called when one of the struct fields is a different struct. This is an example:

type ChildStruct struct {
    Value int
}

type ParentStruct struct {
    Child ChildStruct `validate:"myValidate"`
}

func myValidate(fl validator.FieldLevel) bool {
    fmt.Println("INSIDE MY VALIDATOR") // <- This is never printed
    return false
}

func main() {
    validator := validator.New()
    validator.RegisterValidation("myValidate", myValidate)
    data := &ParentStruct{
        Child: ChildStruct{
            Value: 10,
        },
    }
    validateErr := validator.Struct(data)
    if validateErr != nil { // <- This is always nil since MyValidate is never called
        fmt.Println("GOT ERROR")
        fmt.Println(validateErr)
    }
    fmt.Println("DONE")
}

If I change the parentStruct to:

type ParentStruct struct {
    Child int `validate:"myValidate"`
}

everything works. If I add the validate:"myValidate" part to the ChildStruct it is also working, however, then the error that is returned is saying that the ChildStruct.Value is wrong when it should say that the ParentStruct.Child is wrong.

Anyone know what I am doing wrong?

score 6 · Accepted Answer · answered May 03 '21 at 17:23

After searching for a while I finally found a function called RegisterCustomTypeFunc which registers a custom type which will make it possible for go-playground/validator/v10 to validate it. So the solution is to add the following to the example in the question:

func childStructCustomTypeFunc(field reflect.Value) interface{} {   
    if value, ok := field.Interface().(ChildStruct); ok {
        return value.Value
    }
    return nil
}

Together with:

validator.RegisterCustomTypeFunc(childStructCustomTypeFunc, ChildStruct{})

Now the validator will go into the myValidate function and the return message will be an error for the ParentStruct.Child field

score 2 · Answer 2 · answered Apr 26 '21 at 23:38

The function validator.RegisterValidation(...) is registering a field level custom validator, as the type of the registered function suggests func(fl validator.FieldLevel) bool.

Struct fields themselves are not validated this way and your custom validator is ignored.

To validate a struct field you should use validate.RegisterStructValidation(myValidate, ChildStruct{}), where the function myValidate is of type validator.StructLevelFunc.

Inside this function you can then perform validation of the struct, either the field itself and/or its nested fields:

func myValidate(sl validator.StructLevel) {
    fmt.Println("INSIDE MY VALIDATOR") // now called
    if sl.Current().Interface().(ChildStruct).Value != 20 {
        sl.ReportError(sl.Current().Interface(), "ChildStruct", "", "", "")
    }
}

func main() {
    vald := validator.New()
    vald.RegisterStructValidation(myValidate, ChildStruct{})
    data := &ParentStruct{
        Child: ChildStruct{
            Value: 10,
        },
    }
    validateErr := vald.Struct(data)
    if validateErr != nil {         
        fmt.Println("GOT ERROR")
        fmt.Println(validateErr)
    }
    fmt.Println("DONE")
}

Example on the playground: https://play.golang.org/p/f0f2YE_e1VL

How can I validate a struct datatype with a custom validator?

2 Answers2