added validation when adding a book

internal/api/validator.go

@@ -0,0 +1,35 @@
+package api
+
+import (
+	"fmt"
+
+	"github.com/go-playground/validator/v10"
+)
+
+type apiValidationError struct {
+	Field string `json:"field"`
+	Err   string `json:"error"`
+}
+
+func getValidationErrors(ve *validator.ValidationErrors) []apiValidationError {
+	errors := make([]apiValidationError, len(*ve))
+	for i, fe := range *ve {
+		errors[i] = apiValidationError{
+			Field: fe.Field(),
+			Err:   computeValidationMessage(&fe),
+		}
+	}
+	return errors
+}
+
+func computeValidationMessage(fe *validator.FieldError) string {
+	tag := (*fe).Tag()
+	switch tag {
+	case "required":
+		return fmt.Sprintf("%s is required.", (*fe).Field())
+	case "max":
+		return fmt.Sprintf("%s is too long. It should be under %s characters.", (*fe).Field(), (*fe).Param())
+	default:
+		return fmt.Sprintf("Validation failed for '%s' property.", tag)
+	}
+}