Issue

The below validator works for invalid request body, but for a valid body, it initiates a panic

Code sample, to showcase or reproduce:

data/validation.go

func (v *Validation) Validate(i interface{}) ValidationErrors {
	errs := v.validate.Struct(i).(validator.ValidationErrors)

	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
}

handlers/post.go :

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/products.go

// 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"`
}

Stack Trace:

products-api 2020/07/04 17:04:27 http: panic serving 127.0.0.1:47468: interface conversion: interface {} is *data.Product, not data.Product
goroutine 50 [running]:
net/http.(*conn).serve.func1(0xc0005fe0a0)
        /usr/local/go/src/net/http/server.go:1772 +0x139
panic(0x951fe0, 0xc00030a480)
        /usr/local/go/src/runtime/panic.go:973 +0x3e3
github.com/nicholasjackson/building-microservices-youtube/product-api/handlers.(*Products).Create(0xc0003192c0, 0xaa9320, 0xc00046a000, 0xc000400400)
        /tmp/building-microservices-youtube/product-api/handlers/post.go:20 +0x349
net/http.HandlerFunc.ServeHTTP(0xc000319510, 0xaa9320, 0xc00046a000, 0xc000400400)
        /usr/local/go/src/net/http/server.go:2012 +0x44
github.com/nicholasjackson/building-microservices-youtube/product-api/handlers.(*Products).MiddlewareValidateProduct.func1(0xaa9320, 0xc00046a000, 0xc000400200)
        /tmp/building-microservices-youtube/product-api/handlers/middleware.go:42 +0x553
net/http.HandlerFunc.ServeHTTP(0xc00027c0a0, 0xaa9320, 0xc00046a000, 0xc000400200)
        /usr/local/go/src/net/http/server.go:2012 +0x44
github.com/gorilla/mux.(*Router).ServeHTTP(0xc000156300, 0xaa9320, 0xc00046a000, 0xc000400000)
        /home/ayman/go/pkg/mod/github.com/gorilla/[email protected]/mux.go:212 +0xe2
net/http.serverHandler.ServeHTTP(0xc00055c1c0, 0xaa9320, 0xc00046a000, 0xc000400000)
        /usr/local/go/src/net/http/server.go:2807 +0xa3
net/http.(*conn).serve(0xc0005fe0a0, 0xaaa460, 0xc00021c040)
        /usr/local/go/src/net/http/server.go:1895 +0x86c
created by net/http.(*Server).Serve
        /usr/local/go/src/net/http/server.go:2933 +0x35c

Could it be the version 10 from go validator has broken the implementation? I have cloned your repo and checked out episode_7 branch and it throws the above error for valid requests.

Was facing the same issue - able to resolve it by following the solution - is it just because we are casting the error to nil ?

 errs := v.validate.Struct(i).(validator.ValidationErrors)

According to blow link this should work ?

https://stackoverflow.com/questions/29138591/hiding-nil-values-understanding-why-golang-fails-here/29138676#29138676

// Refer: https://github.com/go-playground/validator/issues/632
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
}

@nicholasjackson can you please update the Validate function under data/validation.go (from episode_7 branch and above)