title: Django post save signal not updating the database
body: <p>Im not getting why this signal is not working. This same code worked once but after that i deleted the objects from admin and ran it again and it stopped working.</p>
  
  <pre><code>@receiver(post_save, sender=FinancePending)
  def calcualate_FinancePending(sender, instance, created, **kwargs):
  amount_paid = FinancePending.objects.values_list('amountPaid', flat=True)
  amount_paid = list(amount_paid)
  total_amount = FinancePending.objects.values_list('TotalAmount', flat=True)
  total_amount = list(total_amount)
  
  # total - paid
  TotalFee = [int(s.replace(',', '')) for s in total_amount]
  AmountPaid = [int(s.replace(',', '')) for s in amount_paid]
  finance_pending = FinancePending.objects.all()
  i = 1
  
  while i &lt;= len(TotalFee):
      amount_pending = TotalFee[i-1] - AmountPaid[i-1]
      amountpending = FinancePending.objects.filter(invoiceNumber=i)
      amountpending.AmountPending = amount_pending
      i = 1 + i
  </code></pre>