Fix discount usage verification

insalan/payment/models.py

@@ -340,7 +340,7 @@ def validate_transaction(self):
         # For each discount, mark it as used
         for discount in self.discounts.all():
             discount.use()
-        
+
         self.save()
         logger.info("Transaction %s succeeded", self.id)
         self.run_success_hooks()
@@ -394,6 +394,10 @@ class Meta:
     )
     count = models.IntegerField(default=1, editable=True, verbose_name=_("Quantité"))
 
+# Discount
+
+class DiscountAlreadyUsedError(Exception):
+    """Error raised when trying to use an already used discount"""
 
 class Discount(models.Model):
     """
@@ -434,6 +438,8 @@ class Meta:
 
     def use(self):
         """Use the discount"""
+        if self.used:
+            raise DiscountAlreadyUsedError("Discount already used")
         self.used = True
         self.used_date = timezone.make_aware(datetime.now())
         self.save()

insalan/payment/views.py

@@ -429,7 +429,7 @@ def create(self, request):
 
             # If the user has a discount for some products, apply them
             for product in transaction_obj.products.all():
-                discounts = Discount.objects.filter(user=payer, product=product)
+                discounts = Discount.objects.filter(user=payer, product=product, used=False)
                 if discounts.exists():
                     discount = discounts.first()
                     amount = transaction_obj.amount - discount.discount