Getting a specific element from a queryset in an post_delete returns None

[megasnort]

I encountered the following bug when using

• Django 2.0.4
• django-polymorphic 2.0.2
• Python 3.6.4
• Postgresql and sqllite

Bug description

Create two instances of Classes that have the same Base class, using django-polymorphic. The Class of the first (!) instance has a post_delete signal

In this post_delete signal, you issue a Query to the database to determine what is the 'first' element of all the existing instances of the base class.

Now, delete the first instance, the one with the signal.

If you do it like this, it does not work, because the [0] always returns None. If should however show the other instance.

try:
    print(Entry.objects.order_by('pk')[0])
except IndexError:
    pass

if you do it like this, it also does not work, because the first() also always returns None. (first() is an easier form of [0] with the try/except)

print(Entry.objects.order_by('pk').first())

If you do it like this, it does work!

entry = None
for entry in Entry.objects.order_by('pk'):
    print(entry)
    break

Note that in all three case, when you issue

print(Entry.objects.order_by('pk'))

You do see the Queryset, with the element that was not deleted.

Notes

• The order in which the instances are created does matter (if your create the item with the signal secondly, the bug does not occur)
• The problem also occurs when using 'last()'

Example code

I created a repo with tests that trigger the bug like mentioned above: https://github.com/megasnort/django_polymorphic_bug_first_last_post_delete

pip install -r requirements.txt`
cd django_first_bug
python manage.py test

You will see that two tests fail, that should pass. They don't because the first() and [0] method of the queryset return None instead of the first element.

[pipanchik]

I have the same problem