Re: DoesNotExist on ForeignKey Access

Some more code will be helpful, for example the model structure, how do you define the the `ForeignKey`? How do you subscribe for the signal?
You are saying that with sleep(1) the code works fine, is this mean that you can reproduce the problem at any time?
However test it without fetching the blob object, this would probably fix the problem:



if widget.blob_id is not None and Widget.objects.filter(blob_id=widget.blob_id).count() == 0:
    Blob.objects.filter(id=widget.blob_id).delete()

On Friday, August 12, 2016 at 9:00:58 PM UTC+3, TheBeardedTemplar wrote:

Hey all,

I'm quite stumped with a problem that recently started showing up in my code. I have a model called a Widget that can have an associated Blob object. When a Widget is deleted, I want to make sure that the Blob associated with it isn't used elsewhere, and if it's not, delete it. I do that via a postDelete hook, as follows:

def postDeleteWidget(sender, **kwargs):
    """
    Delete the associated blob when the widgetis deleted.
    """
    widget = kwargs['instance']


    #Make sure this blob isn't used elsewhere
    if widget.blob is not None and Widget.objects.filter(blob=widget.blob).count() == 0:
        widget.blob.delete()

This come is failing when it first looks up widget.blob in these Django files:

File "C:\Python27\Lib\site-packages\django\db\models\fields\related.py", line 617, in __get__
  rel_obj = qs.get()
File "C:\Python27\Lib\site-packages\django\db\models\query.py", line 334, in get
  self.model._meta.object_name

myproject.apps.blobs.models.DoesNotExist: Blob matching query does not exist.

Now here's the weird part. If I insert "sleep(1)" after getting the widget, the code works fine. It deletes the associated blob as expected. I'm really at a loss in terms of how to explain this. I'd appreciate any advice. Even printing 'widget.blob' before the if statement usually resolves it. Is this some kind of race condition involved in accessing a related key on a postDelete hook.

Thanks,

-Keilan

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