pre_save is dispatched before executing upload_to attributes on FileFields when using ModelForm

[Peter Bengtsson]

Suppose you have this code:

def callable_upload_to(instance, filename):
    full_path = os.path.join(this_year(), filename)
    print "PLACING", full_path
    return full_path

class Foo(models.Model):
    file = models.FileField(upload_to=callable_upload_to)
    size = models.IntegerField(null=True)

def update_foo(sender, instance, **kwargs):
    print "UPDATING", str(instance.file)

pre_save.connect(update_foo, sender=Foo)

Suppose you create one of these instances with a ModelForm you'll get this output on stdout:

UPDATING sample.file
PLACING 2010/sample.file

That means that you can't rely on the file path as dictated by the upload_to callable in your pre_save signal. Especially important is that within your pre_save method you can't even get to the file since it doesn't exist.

[Peter Bengtsson]

I suspect that the culprit is Model.save_base in django/db/models/base.py where it dispatches the signal before it does whatever it does to those meta.local_files but I don't understand the code enough to attempt a patch at this point.

[Karen Tracey]

This behavior got a fair amount of discussion when it was introduced. See #10788 and the linked threads on django-developers.

[Peter Bengtsson]

Oh I see. I really did google the tickets but didn't find that one.

Personally thinks it sucks but will keep my mouth shut till I provide a patch of my own :)