From 7e08a07e29c16219a5a591f1b44b8ff2630bc086 Mon Sep 17 00:00:00 2001
From: EUForest <EUForest@tutamail.com>
Date: Fri, 6 Feb 2026 23:14:04 +0800
Subject: [PATCH] fix: resolve balance payment issue with coupon and fee
 calculation

This commit fixes the inconsistent calculation logic between order preview
and actual order creation, which caused balance payment failures when using
coupons.

Changes:
- Standardized fee calculation order in both preCreateOrderLogic and purchaseLogic
- Moved gift amount deduction after fee calculation to ensure correct total
- Removed premature gift amount deduction before transaction in purchaseLogic
- Gift amount is now only deducted within the database transaction

The calculation order is now unified:
1. Apply coupon discount
2. Calculate handling fee based on post-coupon amount
3. Deduct gift amount from total (including fee)

This ensures the preview amount matches the actual payment amount.
---
 .../logic/public/order/preCreateOrderLogic.go | 25 ++++++++--------
 internal/logic/public/order/purchaseLogic.go  | 29 +++++++++----------
 2 files changed, 26 insertions(+), 28 deletions(-)

diff --git a/internal/logic/public/order/preCreateOrderLogic.go b/internal/logic/public/order/preCreateOrderLogic.go
index 7c86330..e0e48a4 100644
--- a/internal/logic/public/order/preCreateOrderLogic.go
+++ b/internal/logic/public/order/preCreateOrderLogic.go
@@ -117,18 +117,6 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		couponAmount = calculateCoupon(amount, couponInfo)
 	}
 	amount -= couponAmount
-
-	var deductionAmount int64
-	// Check user deduction amount
-	if u.GiftAmount > 0 {
-		if u.GiftAmount >= amount {
-			deductionAmount = amount
-			amount = 0
-		} else {
-			deductionAmount = u.GiftAmount
-			amount -= u.GiftAmount
-		}
-	}
 	var feeAmount int64
 	if req.Payment != 0 {
 		payment, err := l.svcCtx.PaymentModel.FindOne(l.ctx, req.Payment)
@@ -139,8 +127,19 @@ func (l *PreCreateOrderLogic) PreCreateOrder(req *types.PurchaseOrderRequest) (r
 		// Calculate the handling fee
 		if amount > 0 {
 			feeAmount = calculateFee(amount, payment)
+			amount += feeAmount
+		}
+	}
+	// Calculate gift amount deduction after fee calculation
+	var deductionAmount int64
+	if u.GiftAmount > 0 && amount > 0 {
+		if u.GiftAmount >= amount {
+			deductionAmount = amount
+			amount = 0
+		} else {
+			deductionAmount = u.GiftAmount
+			amount -= u.GiftAmount
 		}
-		amount += feeAmount
 	}
 
 	resp = &types.PreOrderResponse{
diff --git a/internal/logic/public/order/purchaseLogic.go b/internal/logic/public/order/purchaseLogic.go
index 06f611c..67769ea 100644
--- a/internal/logic/public/order/purchaseLogic.go
+++ b/internal/logic/public/order/purchaseLogic.go
@@ -130,19 +130,6 @@ func (l *PurchaseLogic) Purchase(req *types.PurchaseOrderRequest) (resp *types.P
 	}
 	// Calculate the handling fee
 	amount -= coupon
-	var deductionAmount int64
-	// Check user deduction amount
-	if u.GiftAmount > 0 {
-		if u.GiftAmount >= amount {
-			deductionAmount = amount
-			amount = 0
-			u.GiftAmount -= deductionAmount
-		} else {
-			deductionAmount = u.GiftAmount
-			amount -= u.GiftAmount
-			u.GiftAmount = 0
-		}
-	}
 	// find payment method
 	payment, err := l.svcCtx.PaymentModel.FindOne(l.ctx, req.Payment)
 	if err != nil {
@@ -155,6 +142,17 @@ func (l *PurchaseLogic) Purchase(req *types.PurchaseOrderRequest) (resp *types.P
 		feeAmount = calculateFee(amount, payment)
 		amount += feeAmount
 	}
+	// Calculate gift amount deduction after fee calculation
+	var deductionAmount int64
+	if u.GiftAmount > 0 && amount > 0 {
+		if u.GiftAmount >= amount {
+			deductionAmount = amount
+			amount = 0
+		} else {
+			deductionAmount = u.GiftAmount
+			amount -= u.GiftAmount
+		}
+	}
 	// query user is new purchase or renewal
 	isNew, err := l.svcCtx.OrderModel.IsUserEligibleForNewOrder(l.ctx, u.Id)
 	if err != nil {
@@ -200,9 +198,10 @@ func (l *PurchaseLogic) Purchase(req *types.PurchaseOrderRequest) (resp *types.P
 			}
 		}
 
-		// update user deduction && Pre deduction ,Return after canceling the order
+		// update user gift amount and create deduction record
 		if orderInfo.GiftAmount > 0 {
-			// update user deduction && Pre deduction ,Return after canceling the order
+			// deduct gift amount from user
+			u.GiftAmount -= orderInfo.GiftAmount
 			if e := l.svcCtx.UserModel.Update(l.ctx, u, db); e != nil {
 				l.Errorw("[Purchase] Database update error", logger.Field("error", e.Error()), logger.Field("user", u))
 				return e