fix(backend): check user duplicate discount usage for approved payments

backend/backend_api/admin.py

@@ -103,7 +103,7 @@ def execute_mailer(self, request, obj):
 
 
 class DiscountAdmin(admin.ModelAdmin):
-    list_display = ('__str__', 'is_active', 'discount_percent', 'capacity', 'expiration_date')
+    list_display = ('__str__', 'is_active', 'discount_percent', 'capacity', 'remaining_capacity', 'expiration_date')
     readonly_fields = ('participants',)
 
     class Meta:

backend/backend_api/models.py

@@ -362,20 +362,21 @@ def participants(self):
             users.append(payment.user)
         return users
 
-    def _remaining_capacity(self) -> int:
+    @property
+    def remaining_capacity(self) -> int:
         return self.capacity - self.payment_set.filter(status=Payment.PaymentStatus.PAYMENT_CONFIRMED).count()
 
     def is_usable(self, user: User) -> bool:
         if not self.is_active:
             raise ValidationError(new_detailed_response(status.HTTP_400_BAD_REQUEST,
                                                         "Discount is not active"))
-        if self._remaining_capacity() <= 0:
+        if self.remaining_capacity <= 0:
             raise ValidationError(new_detailed_response(status.HTTP_400_BAD_REQUEST,
                                                         "Discount capacity is full"))
         if self.expiration_date < timezone.now():
             raise ValidationError(new_detailed_response(status.HTTP_400_BAD_REQUEST,
                                                         "Discount has expired"))
-        if self.payment_set.filter(user=user).exists():
+        if self.payment_set.filter(user=user, status=Payment.PaymentStatus.PAYMENT_CONFIRMED).exists():
             raise ValidationError(new_detailed_response(status.HTTP_400_BAD_REQUEST,
                                                         "Discount has already been used by this user"))
         return True