id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
27982	Possible race condition related to queryset union	gigelu	nobody	"I found a strange bug: the results from a paginator are affected randomly by setting breakpoints in the IDE.

The bug appears '''only''' when I am using `union` on a queryset.

I am using Django 1.11rc1 with DRF 3.6.2.

The models:
{{{
class BaseNotification(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    type = models.CharField(max_length=10, choices=TYPES, default=TYPE_SIMPLE, blank=True)
    popup_type = models.CharField(max_length=10, choices=POPUP_TYPES, default=POPUP_DEFAULT)
    title = models.CharField(max_length=200, blank=True)
    message = models.TextField()
    completed = models.BooleanField(blank=True, default=False)
    date_completed = models.DateTimeField(null=True, blank=True, default=None)
    date_created = models.DateTimeField(auto_now_add=True)

class SimpleNotification(BaseNotification):

    def save(self, *args, **kwargs):
        self.type = self.TYPE_SIMPLE
        super().save(*args, **kwargs)


class DecisionNotification(BaseNotification):
	... (not relevant)
}}}

The view:
{{{
class NotificationView(ListAPIView):
    permission_classes = [IsAuthenticated]
    filter_backends = [OrderingFilter]
    ordering_fields = ['completed', 'date_created', 'type']
    ordering = ['-completed', '-date_created']
    serializer_class = NotificationSerializer

    def get_queryset(self):
        qs1 = SimpleNotification.objects.filter(user=self.request.user).only(
            'type', 'popup_type', 'completed', 'date_created')
        qs2 = DecisionNotification.objects.filter(user=self.request.user).only(
            'type', 'popup_type', 'completed', 'date_created')
        qs = qs1.union(qs2)
        return qs
}}}

The serializer:
{{{
class NotificationSerializer(serializers.ModelSerializer):
    title = serializers.CharField(source='the_title')

    class Meta:
        model = SimpleNotification
        fields = ['type', 'popup_type', 'title', 'url', 'completed', 'date_created']
}}}

The problem: I have 5 notifications in DB (3 simple, 2 decision), but it returns only the first one in normal usage, and sometimes all or sometimes one when I am using breakpoints. 

I've isolated the problem near this line of code: https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/pagination.py#L208. If I put a breakpoint inside the paginator's init (https://github.com/django/django/blob/master/django/core/paginator.py#L29) I get the correct result every time.

http://i.imgur.com/9zoikMQ.png
http://i.imgur.com/bsHgxig.png

Here's a short video (1m 35s, 11.6MB) https://www.dropbox.com/s/72u46vx2n0y9h9y/bug.webm?dl=0
"	Bug	new	Database layer (models, ORM)	1.11	Normal				Unreviewed	0	0	0	0	0	0