Make vendor nullable

internal/transformer/purchase_order_transformer_test.go

@@ -12,12 +12,13 @@ import (
 )
 
 func TestCreatePurchaseOrderRequestToModelAllowsMissingDueDate(t *testing.T) {
+	vendorID := uuid.New()
 	ingredientID := uuid.New()
 	quantity := 1.0
 	unitID := uuid.New()
 
 	result, err := CreatePurchaseOrderRequestToModel(&contract.CreatePurchaseOrderRequest{
-		VendorID:        uuid.New(),
+		VendorID:        &vendorID,
 		PONumber:        "PO-001",
 		TransactionDate: "2026-05-29",
 		Items: []contract.CreatePurchaseOrderItemRequest{
@@ -35,9 +36,10 @@ func TestCreatePurchaseOrderRequestToModelAllowsMissingDueDate(t *testing.T) {
 }
 
 func TestPurchaseOrderModelResponseToResponseIncludesNullDueDate(t *testing.T) {
+	vendorID := uuid.New()
 	result := PurchaseOrderModelResponseToResponse(&models.PurchaseOrderResponse{
 		ID:       uuid.New(),
-		VendorID: uuid.New(),
+		VendorID: &vendorID,
 		PONumber: "PO-001",
 	})
 
@@ -45,3 +47,19 @@ func TestPurchaseOrderModelResponseToResponseIncludesNullDueDate(t *testing.T) {
 	require.NoError(t, err)
 	require.Contains(t, string(payload), `"due_date":null`)
 }
+
+func TestCreatePurchaseOrderRequestToModelAllowsMissingVendor(t *testing.T) {
+	result, err := CreatePurchaseOrderRequestToModel(&contract.CreatePurchaseOrderRequest{
+		PONumber:        "PO-001",
+		TransactionDate: "2026-05-29",
+		Items: []contract.CreatePurchaseOrderItemRequest{
+			{
+				PurchaseCategoryID: uuid.New(),
+				Amount:             1000,
+			},
+		},
+	})
+
+	require.NoError(t, err)
+	require.Nil(t, result.VendorID)
+}