Interface conversion: interface {} is nil, not validator.ValidationErrors - validation

Question

I am trying to create a CRUD and validating my request body using a Go library called [validator][1]
Validation Code sample:
func (v *Validation) Validate(i interface{}) ValidationErrors {
errs := v.validate.Struct(i).(validator.ValidationErrors) // panic originates here
if len(errs) == 0 {
return nil
}
var returnErrs []ValidationError
for _, err := range errs {
// cast the FieldError into our ValidationError and append to the slice
ve := ValidationError{err.(validator.FieldError)}
returnErrs = append(returnErrs, ve)
}
return returnErrs
}
The above validator works for invalid request body such as invalid ID.
But for a valid body, it initiates a panic
Stack Trace:
products-api 2020/07/12 15:15:11 http: panic serving 127.0.0.1:33288: interface conversion: error is nil, not validator.ValidationErrors
goroutine 21 [running]:
net/http.(*conn).serve.func1(0xc0003b6140)
/usr/local/go/src/net/http/server.go:1772 +0x139
panic(0x93d6c0, 0xc0003845d0)
/usr/local/go/src/runtime/panic.go:973 +0x3e3
github.com/AymanArif/golang-microservices/data.(*Validation).Validate(0xc0005a8108, 0x8f3480, 0xc0005aa2c0, 0x0, 0x0, 0x203000)
/home/ayman/Desktop/golang-microservices/data/validation.go:70 +0x211
interface conversion: interface conversion: error is nil, not validator.ValidationErrors
Create REST Logic:
func (p *Products) Create(rw http.ResponseWriter, r *http.Request) {
prod := r.Context().Value(KeyProduct{}).(data.Product) // Panic originate here. Check below for struct definiton
p.l.Printf("[DEBUG] Inserting product: %#v\n", prod)
data.AddProduct(prod)
}
// data.Product
type Product struct {
ID int `json:"id"` // Unique identifier for the product
Name string `json:"name" validate:"required"`
Description string `json:"description"`
SKU string `json:"sku" validate:"sku"`
}
How can I do error handling for correct requests?
[1]: https://github.com/go-playground/validator

Answer 1

The error you got is eloquent:
interface conversion: interface {} is *data.Product, not data.Product
The line r.Context().Value(KeyProduct{}) returns an interface type interface{}, which, the error tells you, is holding a value of concrete type *data.Product (pointer to data.Product)
Instead, you are attempting to convert it to a data.Product, without checking whether the conversion is valid.
Replace the line with:
prod := r.Context().Value(KeyProduct{}).(*data.Product)
You might want to read the Go Tour about type assertions.
After your update, the error you have now is still the same kind of issue:
interface conversion: error is nil, not validator.ValidationErrors
With the expression err.(validator.FieldError) you are attempting to convert err to a validator.FieldError when err is in fact nil.
The previous check len(errs) == 0 is only verifying that errs length is not zero, but the slice might be of non-zero length and contain nil values.
You can instead test the type assertion:
if fe, ok := err.(validator.FieldError); ok {
ve := ValidationError{fe}
returnErrs = append(returnErrs, ve)
}

Answer 2

If you re write the Validate function to something like this it should work. Type assertion needs to be checked for errs & err
func (v *Validation) Validate(i interface{}) ValidationErrors {
var returnErrs []ValidationError
if errs, ok := v.validate.Struct(i).(validator.ValidationErrors); ok {
if errs != nil {
for _, err := range errs {
if fe, ok := err.(validator.FieldError); ok {
ve := ValidationError{fe}
returnErrs = append(returnErrs, ve)
}
}
}
}
return returnErrs
}