Fix loading sheet creation flow

Distribution/Features/LoadingEvent/LoadingSheetService.cs

@@ -64,26 +64,47 @@ public class LoadingSheetService(
         if (sheet.Status != LoadingSheetStatus.Planned)
             throw new InvalidOperationException("Items can only be changed while the sheet is Planned");
 
-        var shopPOItemIds = request.Items
-            .Where(i => i.FK_ShopPOItemId.HasValue)
-            .Select(i => i.FK_ShopPOItemId!.Value)
-            .ToList();
-        var shopPOIds = shopPOItemIds.Count > 0
-            ? await repo.GetShopPOIdsByItemIdsAsync(shopPOItemIds)
-            : new List<Guid>();
-
+        // Upsert by (item, warehouse, shop PO item): a matching row gets its quantity
+        // updated in place, a new combination is added. Rows for combinations not present
+        // in this request are left untouched, so a later save can top up the same PO
+        // item's remaining quantity from a different warehouse without losing what was
+        // already planned from the first one.
         await uow.BeginAsync(ct);
         try
         {
-            repo.RemoveItems(sheet.Items);
-            await repo.AddItemsAsync(request.Items.Select(i => new LoadingSheetItem
+            var newItems = new List<LoadingSheetItem>();
+            foreach (var input in request.Items)
             {
-                FK_LoadingSheetId = sheet.Id,
-                FK_ItemId = i.FK_ItemId,
-                FK_WarehouseId = i.FK_WarehouseId,
-                FK_ShopPOItemId = i.FK_ShopPOItemId,
-                QuantityDispatched = i.Quantity
-            }));
+                var existing = sheet.Items.FirstOrDefault(i =>
+                    i.FK_ItemId == input.FK_ItemId &&
+                    i.FK_WarehouseId == input.FK_WarehouseId &&
+                    i.FK_ShopPOItemId == input.FK_ShopPOItemId);
+
+                if (existing is not null)
+                    existing.QuantityDispatched = input.Quantity;
+                else
+                    newItems.Add(new LoadingSheetItem
+                    {
+                        FK_LoadingSheetId = sheet.Id,
+                        FK_ItemId = input.FK_ItemId,
+                        FK_WarehouseId = input.FK_WarehouseId,
+                        FK_ShopPOItemId = input.FK_ShopPOItemId,
+                        QuantityDispatched = input.Quantity
+                    });
+            }
+            if (newItems.Count > 0)
+                await repo.AddItemsAsync(newItems);
+
+            var shopPOItemIds = sheet.Items.Select(i => i.FK_ShopPOItemId)
+                .Concat(newItems.Select(i => i.FK_ShopPOItemId))
+                .Where(x => x.HasValue)
+                .Select(x => x!.Value)
+                .Distinct()
+                .ToList();
+            var shopPOIds = shopPOItemIds.Count > 0
+                ? await repo.GetShopPOIdsByItemIdsAsync(shopPOItemIds)
+                : new List<Guid>();
+
             await repo.ReplaceOrdersAsync(sheet.Id, shopPOIds);
             await uow.CommitAsync(ct);
         }