Add validation of single tax from each class

We only allow a single tax of each class in products and invoices,
because it does not make sense to add two different VAT to the same
product, for instance.

pkg/form.go

@@ -127,13 +127,17 @@ func (field *SelectField) IsSelected(v string) bool {
 	return false
 }
 
-func (field *SelectField) isValidOption(selected string) bool {
+func (field *SelectField) FindOption(value string) *SelectOption {
 	for _, option := range field.Options {
-		if option.Value == selected {
+		if option.Value == value {
-			return true
+			return option
 		}
 	}
-	return false
+	return nil
+}
+
+func (field *SelectField) isValidOption(selected string) bool {
+	return field.FindOption(selected) != nil
 }
 
 func MustGetOptions(ctx context.Context, conn *Conn, sql string, args ...interface{}) []*SelectOption {
@@ -225,6 +229,20 @@ func (v *FormValidator) CheckValidSelectOption(field *SelectField, message strin
 	return v.checkSelect(field, field.HasValidOptions(), message)
 }
 
+func (v *FormValidator) CheckAtMostOneOfEachGroup(field *SelectField, message string) bool {
+	repeated := false
+	groups := map[string]bool{}
+	for _, selected := range field.Selected {
+		option := field.FindOption(selected)
+		if exists := groups[option.Group]; exists {
+			repeated = true
+			break
+		}
+		groups[option.Group] = true
+	}
+	return v.checkSelect(field, !repeated, message)
+}
+
 func (v *FormValidator) CheckValidURL(field *InputField, message string) bool {
 	_, err := url.ParseRequestURI(field.Val)
 	return v.checkInput(field, err == nil, message)

pkg/invoices.go

@@ -525,5 +525,6 @@ func (form *invoiceProductForm) Validate() bool {
 		validator.CheckValidInteger(form.Discount, 0, 100, gettext("Discount must be a percentage between 0 and 100.", form.locale))
 	}
 	validator.CheckValidSelectOption(form.Tax, gettext("Selected tax is not valid.", form.locale))
+	validator.CheckAtMostOneOfEachGroup(form.Tax, gettext("You can only select a tax of each class.", form.locale))
 	return validator.AllOK()
 }

pkg/products.go

@@ -208,5 +208,6 @@ func (form *productForm) Validate() bool {
 		validator.CheckValidDecimal(form.Price, form.company.MinCents(), math.MaxFloat64, gettext("Price must be a number greater than zero.", form.locale))
 	}
 	validator.CheckValidSelectOption(form.Tax, gettext("Selected tax is not valid.", form.locale))
+	validator.CheckAtMostOneOfEachGroup(form.Tax, gettext("You can only select a tax of each class.", form.locale))
 	return validator.AllOK()
 }