Model objects with deferred fields don't send pre_save or post_save signals to receivers with explicit sender

[Evan Heidtmann]

Because the objects returned by Model.objects.only(*fields) are of a special object type, and that type is used as the sender for the post_save signal send, these deferred objects never invoke signal handlers that registered with an explicit signal sender.

Something like this should reproduce:

# models.py
class MyModel(models.Model):
     bigfield = models.TextField(null=True, blank=True)

# signals.py
@receiver.register(post_save, sender=models.MyModel)
def mymodel_post_save_handler(sender, **kwargs):
     print "handler called"

# repro code
m = MyModel.objects.create()
# (handler is called)
deferred_m = MyModel.objects.only('id').get(id=m.id)
deferred_m.bigfield = 'yo'
deferred_m.save()
# (handler should be called, but is not)

Happy to produce a proper test app/project if needed.

Evan

[Simon Charette]

Hi ezheidtmann,

You've reported this bug against Django 1.10 but this should have been solved by 7f51876f99851fdc3fef63aecdfbcffa199c26b9 (#26207) which stopped creating custom proxy classes for deferred fields and which is part of 1.10. The deletion signal part was tested to be fixed in #18100.

Please reopen this ticket if you're still experiencing this issue with Django 1.10a1.