fix: only support qty=1 for flat fee lines

openmeter/billing/invoiceline.go

@@ -515,8 +515,19 @@ func (i Line) ValidateFee() error {
 		errs = append(errs, errors.New("price should be positive or zero"))
 	}
 
-	if !i.FlatFee.Quantity.IsPositive() {
-		errs = append(errs, errors.New("quantity should be positive required"))
+	if i.Status == InvoiceLineStatusValid {
+		// Valid lines (top level invoice lines must have qty=1)
+		// Given product catalog only supports 1 as quantity, let's restrict ourselves to 1 too
+		// as the product catalog spec drives billing behavior
+		if !i.FlatFee.Quantity.Equal(alpacadecimal.NewFromInt(1)) {
+			errs = append(errs, errors.New("quantity should be 1 for invoice's flat fee lines"))
+		}
+	} else {
+		// detailed lines can have any quantity, but not negative (e.g. unit based price's qty is the
+		// usage)
+		if !i.FlatFee.Quantity.IsPositive() {
+			errs = append(errs, errors.New("quantity should be positive"))
+		}
 	}
 
 	if !slices.Contains(FlatFeeCategory("").Values(), string(i.FlatFee.Category)) {

test/app/custominvoicing/invocing_test.go

@@ -191,7 +191,7 @@ func (s *CustomInvoicingTestSuite) TestInvoicingFlowHooksEnabled() {
 						Name: "Test item - HUF",
 
 						PerUnitAmount: alpacadecimal.NewFromFloat(200),
-						Quantity:      alpacadecimal.NewFromFloat(3),
+						Quantity:      alpacadecimal.NewFromFloat(1),
 						PaymentTerm:   productcatalog.InAdvancePaymentTerm,
 					}),
 					{
@@ -359,8 +359,8 @@ func (s *CustomInvoicingTestSuite) TestInvoicingFlowPaymentStatusOnly() {
 
 						Name: "Test item - HUF",
 
-						PerUnitAmount: alpacadecimal.NewFromFloat(200),
-						Quantity:      alpacadecimal.NewFromFloat(3),
+						PerUnitAmount: alpacadecimal.NewFromFloat(600),
+						Quantity:      alpacadecimal.NewFromFloat(1),
 						PaymentTerm:   productcatalog.InAdvancePaymentTerm,
 					}),
 				},

test/billing/invoice_test.go

@@ -178,7 +178,7 @@ func (s *InvoicingTestSuite) TestPendingLineCreation() {
 						Name: "Test item - HUF",
 
 						PerUnitAmount: alpacadecimal.NewFromFloat(200),
-						Quantity:      alpacadecimal.NewFromFloat(3),
+						Quantity:      alpacadecimal.NewFromFloat(1),
 						PaymentTerm:   productcatalog.InAdvancePaymentTerm,
 					}),
 					{
@@ -493,7 +493,7 @@ func (s *InvoicingTestSuite) TestCreateInvoice() {
 					Name: "Test item2",
 
 					PerUnitAmount: alpacadecimal.NewFromFloat(200),
-					Quantity:      alpacadecimal.NewFromFloat(3),
+					Quantity:      alpacadecimal.NewFromFloat(1),
 					PaymentTerm:   productcatalog.InAdvancePaymentTerm,
 				}),
 			},

test/billing/suite.go

@@ -367,7 +367,7 @@ func (s *BaseSuite) CreateGatheringInvoice(t *testing.T, ctx context.Context, in
 					},
 					FlatFee: &billing.FlatFeeLine{
 						PerUnitAmount: alpacadecimal.NewFromFloat(200),
-						Quantity:      alpacadecimal.NewFromFloat(3),
+						Quantity:      alpacadecimal.NewFromFloat(1),
 						Category:      billing.FlatFeeCategoryRegular,
 						PaymentTerm:   productcatalog.InAdvancePaymentTerm,
 					},