管理採購單的商品金額修正

resources/js/hooks/usePurchaseOrderForm.ts

@@ -87,7 +87,6 @@ export function usePurchaseOrderForm({ order, suppliers }: UsePurchaseOrderFormP
                 item.previousPrice = product.lastPrice;
 
                 // 決定預設單位
-                // 若有採購單位且等於大單位，預設為大單位
                 const isPurchaseUnitLarge = product.purchase_unit_id && product.large_unit_id && product.purchase_unit_id === product.large_unit_id;
 
                 if (isPurchaseUnitLarge) {
@@ -97,6 +96,9 @@ export function usePurchaseOrderForm({ order, suppliers }: UsePurchaseOrderFormP
                     item.selectedUnit = 'base';
                     item.unitId = product.base_unit_id;
                 }
+
+                // 初始小計 = 數量 * 單價
+                item.subtotal = calculateSubtotal(Number(item.quantity), Number(item.unitPrice));
             }
         } else if (field === "selectedUnit") {
             // @ts-ignore
@@ -106,17 +108,24 @@ export function usePurchaseOrderForm({ order, suppliers }: UsePurchaseOrderFormP
             } else {
                 item.unitId = item.base_unit_id;
             }
+            // Switch unit doesn't change Total Amount (Subtotal), but implies Unit Price changes?
+            // Actually if I switch unit, the Quantity is usually for that unit.
+            // If I have 1 Box ($100), and switch to Pc. Quantity is still 1. 
+            // Total is $100. So Unit Price (per Pc) becomes $100.
+            // This seems safely consistent with "Total Amount" anchor.
         } else {
             // @ts-ignore
             item[field] = value;
         }
 
-        // 計算小計
-        if (field === "quantity" || field === "unitPrice" || field === "productId") {
-            item.subtotal = calculateSubtotal(
-                Number(item.quantity),
-                Number(item.unitPrice)
-            );
+        // 重新計算 (Always derive UnitPrice from Subtotal and Quantity)
+        // 除了剛剛已經算過 subtotal 的 productId case
+        if (field !== "productId") {
+            if (item.quantity > 0) {
+                item.unitPrice = Number(item.subtotal) / Number(item.quantity);
+            } else {
+                item.unitPrice = 0;
+            }
         }
 
         newItems[index] = item;