Possible race condition related to queryset union

[gigelu]

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

I've also attached a video showing the results while using different breakpoints.