queryset.exists() doesn’t work correctly

[ramusus]

I don't understand is it normal?

    In [17]: User.objects.filter(pk=0)
    Out[17]: []

    In [18]: User.objects.filter(pk=0).exists()
    Out[18]: True

The exists() should return False result in this case, I think. This is revision 11653 (Development version)

I have related problem with unique validating of inlineformset and I found that the reason is queryset.exists() doesn't work correctly

[Alex Gaynor]

This works correctly for me.

>>> from django.contrib.auth.models import  User

>>> User.objects.filter(pk=0)
[2] []

>>> User.objects.filter(pk=0).exists()
[3] False

Can you provide some more information about your setup, such as db backend?

[Karen Tracey]

It's failing on MySQL, at least. This code:

​http://code.djangoproject.com/browser/django/trunk/django/db/models/sql/query.py#L2381

simply returns result when the connection can use chunked reads. For mysql result is a callable-interator and bool(<callable-interator>) is True.

[Karen Tracey]

I've attached a possible patch but I don't have much of a clue when it comes to the ORM so someone with more of one in that area should see if there's a better way, etc.

[Karen Tracey]

(No tests needed for this fix...a lookup test added in r11647 is currently failing on backends where this problem occurs.)

[ramusus]

Can you provide some more information about your setup, such as db backend?

DATABASE_ENGINE = 'postgresql_psycopg2'

[Alex Gaynor]

Karen, that patch looks correct. I didn't notice this as Jacob committed a different version than my patch (which included a call to list()).

[Karen Tracey]

(In [11654]) Fixed #12079: Changed has_results to get a single result, thus preventing exists() from always returning True on backends that support chunked reads.

[Jacob]

Milestone 1.2 deleted