post_save error Message' has no attribute

Question

I'm trying to use signals post_save for the first time. I have read the documents, but still need some advice.

I'm trying to update my model field called 'charge'.

@receiver(post_save, sender=Message)
def my_handler(sender, **kwargs):
    if not sender.charge:
        sender(charge=sender.length(sender))
        sender.save()

However, this gives the error Message' has no attribute 'charge', but charge does exist within message!

Answer 1

sender here is the Message class itself, not the instance that's being saved. The actual instance is passed as the keyword argument instance. Also, with post_save, if you're not careful, you'll get yourself in an infinite loop. Better to use pre_save.

@receiver(pre_save, sender=Message)
def my_handler(sender, **kwargs):
    instance = kwargs['instance']
    if not instance.charge:
        instance.charge = instance.length()
        # No need to save, as we're slipping the value in 
        # before we hit the database.