feat: add scheme discount calculation to invoice processing

Distribution/Features/Invoice/InvoiceService.cs

@@ -19,7 +19,7 @@ public class InvoiceService(
             i.Status, i.PaymentStatus, i.CreatedBy,
             i.PrintedAt, i.PrintedBy,
             i.FinalizedAt, i.FinalizedBy,
-            i.TotalAmount,
+            i.TotalAmount, i.DiscountAmount,
             i.Payments?.Where(p => p.IsActive).Sum(p => p.Amount) ?? 0m,
             i.IsActive, i.CreatedAt,
             i.Items.Where(x => x.IsActive).Select(x =>
@@ -30,6 +30,15 @@ public class InvoiceService(
     private static decimal BillableTotal(IEnumerable<InvoiceItem> items) =>
         items.Where(i => i.IsActive && !i.IsRemoved).Sum(i => i.Quantity * i.UnitPrice);
 
+    // TotalAmount = billable lines - scheme discount redeemed on the PO. Re-derived whenever lines change
+    // and at finalize, so a scheme applied after the invoice was drafted is still picked up before it locks.
+    private async Task ApplyTotalsAsync(Models.Invoice invoice, IEnumerable<InvoiceItem> items)
+    {
+        var billable = BillableTotal(items);
+        invoice.DiscountAmount = Math.Min(await repo.GetSchemeDiscountAsync(invoice.FK_ShopPOId), billable);
+        invoice.TotalAmount = billable - invoice.DiscountAmount;
+    }
+
     // Immutable copy of the invoice lines as they stand right now.
     public static InvoiceVersion BuildVersion(Models.Invoice invoice, IEnumerable<InvoiceItem> items, int versionNumber, InvoiceVersionStage stage, Guid userId) =>
         new()
@@ -115,15 +124,15 @@ public class InvoiceService(
             FK_ShopPOId = po.Id,
             Status = InvoiceStatus.Draft,
             PaymentStatus = PaymentStatus.Pending,
-            CreatedBy = createdBy,
-            TotalAmount = po.TotalAmount
+            CreatedBy = createdBy
         };
+        var lines = BuildItemsFromShopPO(po, invoice.Id);
+        await ApplyTotalsAsync(invoice, lines);
 
         await uow.BeginAsync(ct);
         try
         {
             await repo.AddAsync(invoice);
-            var lines = BuildItemsFromShopPO(po, invoice.Id);
             await repo.AddItemsAsync(lines);
             await repo.AddVersionAsync(BuildVersion(invoice, lines, 1, InvoiceVersionStage.Created, createdBy));
             po.InvoiceId = invoice.Id;
@@ -192,7 +201,7 @@ public class InvoiceService(
                 throw new InvalidOperationException("Each edit needs an invoiceItemId (existing line) or an itemId (new line)");
         }
 
-        invoice.TotalAmount = BillableTotal(invoice.Items.Concat(newLines));
+        await ApplyTotalsAsync(invoice, invoice.Items.Concat(newLines));
 
         await uow.BeginAsync(ct);
         try
@@ -223,6 +232,10 @@ public class InvoiceService(
         if (invoice.Status != InvoiceStatus.Draft)
             throw new InvalidOperationException($"Cannot print an invoice in {invoice.Status} status");
 
+        // Printing fixes the amount the customer sees, so pick up any scheme discount applied since the last edit.
+        var lines = invoice.Items.Where(i => i.IsActive).ToList();
+        if (lines.Count > 0) await ApplyTotalsAsync(invoice, lines);
+
         invoice.Status = InvoiceStatus.Printed;
         invoice.PrintedAt = DateTime.Now;
         invoice.PrintedBy = printedBy;
@@ -265,7 +278,7 @@ public class InvoiceService(
         }
 
         if (items.Count > 0)
-            invoice.TotalAmount = BillableTotal(items);
+            await ApplyTotalsAsync(invoice, items);
 
         invoice.Status = InvoiceStatus.Finalized;
         invoice.FinalizedAt = DateTime.Now;