old_contents: 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)
  
new_contents: 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)
  
subject: Fix race condition when tracking num_basket_additions on a voucher