from django.db.models import F
from oscar.apps.basket import signals


def track_voucher_addition(basket, voucher, **kwargs):
    voucher.num_basket_additions += 1
    voucher.__class__._default_manager.filter(pk=voucher.pk).update(
        num_basket_additions=F('num_basket_additions') + 1,
    )


def track_voucher_removal(basket, voucher, **kwargs):
    voucher.num_basket_additions -= 1
    voucher.__class__._default_manager.filter(pk=voucher.pk).update(
        num_basket_additions=F('num_basket_additions') - 1,
    )


signals.voucher_addition.connect(track_voucher_addition)
signals.voucher_removal.connect(track_voucher_removal)

from oscar.apps.basket import signals


def track_voucher_addition(basket, voucher, **kwargs):
    voucher.num_basket_additions += 1
    voucher.save()


def track_voucher_removal(basket, voucher, **kwargs):
    voucher.num_basket_additions -= 1
    voucher.save()


signals.voucher_addition.connect(track_voucher_addition)
signals.voucher_removal.connect(track_voucher_removal)

--- 
+++ 
@@ -1,14 +1,19 @@
+from django.db.models import F
 from oscar.apps.basket import signals
 
 
 def track_voucher_addition(basket, voucher, **kwargs):
     voucher.num_basket_additions += 1
-    voucher.save()
+    voucher.__class__._default_manager.filter(pk=voucher.pk).update(
+        num_basket_additions=F('num_basket_additions') + 1,
+    )
 
 
 def track_voucher_removal(basket, voucher, **kwargs):
     voucher.num_basket_additions -= 1
-    voucher.save()
+    voucher.__class__._default_manager.filter(pk=voucher.pk).update(
+        num_basket_additions=F('num_basket_additions') - 1,
+    )
 
 
 signals.voucher_addition.connect(track_voucher_addition)
Fix race condition when tracking num_basket_additions on a voucher
