如何使用信号来更新Django中的另一个对象？

Question

What I'd like to do is to increase the points of a card when a ScanEvent instance is created. I have the following models:

class Card(models.Model):
    # some attributes

class Points(models.Model):
    benef_card = models.ForeignKey(Card, related_name='points')
    at_owner = models.ForeignKey(OwnerProfile)
    nb_current_points = models.PositiveIntegerField(default=0)

class ScanEvent(Event):
    scanned_card = models.ForeignKey(Card)
    scanned_at_owner = models.ForeignKey(OwnerProfile)
    won_points = models.PositiveIntegerField(default=1)

    # This method is called with a post_save signal to add points to a card
    def add_points_to_card(sender, instance, **kwargs):
        pts = instance.scanned_card.points.get(at_owner=instance.scanned_at_owner)
        pts.nb_current_points += instance.won_points
        pts.save()

# At the end of models.py
signals.post_save.connect(ScanEvent.add_points_to_card,
        sender=ScanEvent)

In a simple Django shell, when I try this:

card = Card.objects.get(pk=2)
event = ScanEvent(scanned_card=card)
ownr = OwnerProfile.objects.get(pk=1);
event.save() # will call add_points_to_card()

I get no error but points are not updated (this is because the signal is not called) (it works when I override the save() method thought)

Thanks.

Answer 1

Not sure if your models are correct. With a ForeignKey from Points to Card, that means each Card has multiple Points instances. So when you have a Card object, you can't access the points directly: card.points is a queryset of all the Points objects that are related to this card.

Either you need to determine somehow which related Points is the one you mean, or change your models. Probably using a OneToOne field instead of a ForeignKey would be more appropriate: then you do only have a single Points instance, and card.points will refer to that instance as you expect and you will then be able to access its nb_current_points value.

Edit

The recursion error is because you have attached your handler to the save signal for all classes, not just ScanEvent. You should be sure to specify the sender parameter when registering it:

signals.post_save.connect(add_points_to_card, sender=ScanEvent)

Note that you'll need to move those out of the ScanEvent class otherwise you'll get name errors.

Answer 2

In your models, a Card may have several Points associated. That is why instance.scanned_card.points is a RelatedManager, not a Point. Try this:

def add_points_to_card(sender, instance, **kwargs):
    for points in instance.scanned_card.points.all():
      points.nb_current_points += instance.won_points