post_save in django to update instance immediately

Question

hello,

I'm trying to immediately update a record after it's saved. This example may seem pointless but imagine we need to use an API after the data is saved to get some extra info and update the record:

def my_handler(sender, instance=False, **kwargs):
    t = Test.objects.filter(id=instance.id)
    t.blah = 'hello'
    t.save()

class Test(models.Model):
    title = models.CharField('title', max_length=200)
    blah = models.CharField('blah', max_length=200)

post_save.connect(my_handler, sender=Test)

So the 'extra' field is supposed to be set to 'hello' after each save. Correct? But it's not working.

Any ideas?

Answer 1

Doesn't the post_save handler take the instance? Why are you filtering using it? Why not just do:

def my_handler(sender, instance=False, created, **kwargs):
  if created:
     instance.blah = 'hello'
     instance.save()

Your existing code doesn't work because it loops, and Test.objects.filter(id=instance.id) returns a query set, not an object. To get a single object directly, use Queryset.get(). But you don't need to do that here. The created argument keeps it from looping, as it only sets it the first time.

In general, unless you absolutely need to be using post_save signals, you should be overriding your object's save() method anyway.

Answer 2

When you find yourself using a post_save signal to update an object of the sender class, chances are you should be overriding the save method instead. In your case, the model definition would look like:

class Test(models.Model):
    title = models.CharField('title', max_length=200)
    blah = models.CharField('blah', max_length=200)

    def save(self, force_insert=False, force_update=False):
        if not self.blah:
            self.blah = 'hello'
        super(Test, self).save(force_insert, force_update)