django.core.Paginator wrong query slicing

[Hristo Trendafilov]

A have a ListView defined like so:

class AllPracticesListView(LoginRequiredMixin, PermissionRequiredMixin, ListView, ToolBoxFunctions):
    template_name = 'practice_list.html'
    model = Practice
    paginate_by = 6
    permission_required = 'customauth.access_practices'

    def get_queryset(self):
        qs = self.get_all_practices_by_user().order_by(
            'company_branch__pk', 'practice_profile__personal_profile__last_name').prefetch_related('practice_profile')
        
        # method < get_all_practices_by_user > is a toolbox method that returns:  
        # Practice.objects.filter(company_branch__owner=self.request.user).order_by('pk')

        return qs

Models are defined like so:

      
 class Practice(models.Model):           
                company_branch = models.ForeignKey(
                         CompanyBranch,
                         on_delete=models.PROTECT,
                         related_name='practices',
                    )                
                # CompanyBranch model has an owner field which ForeignKey to the user model       
                
                practice_profile = models.ForeignKey(
                    PracticeProfile,
                    on_delete=models.PROTECT,
                    related_name='practices',
                )
      
class PracticeProfile(models.Model):               
                    personal_profile = models.ForeignKey(
                        PersonalProfile,
                        on_delete=models.PROTECT,
                        blank=True,
                        null=True,
                    )
                    # PersonalProfile model has a field called < last_name > 

Or schematically:

Practice object relations: 
      -> CompanyBranch 
      -> PracticeProfile 
          -> PersonalProfile /*NULLABLE RELATION/
       

When the code is run, the view does not display results correctly.

I did like so to get the WRONG result:
Added a breakpoint on Paginator._get_page return statement

Queryset is passed okay and is available in
Paginator.object_list

But in the args of Paginator._get_page the queryset is totally different than expected / should be a slice from the first six elements of Paginator.object_list /

I have added a breakpoint like so . I do not use the method itself, just stopped the execution to check what data the Page class itself is called with.

Then I get this result .

As you can see, PKs 1632, 1624, etc. are missing, objects are different/marked red and purple/, and are actually removed and never shown in the view.

If you run slice manually at this point - everything is again fine
This happens only on NULL PersonalProfile objects.

I did like so to get OKAY results:

1. Added a simple breakpoint is added on Paginator.page like so and that breakpoint is just run then the result is okay

2. changed paginate_by to 10

3. changed order_by in get_queryset to -pk

4. call again the page = paginator.page(page_number)

P.S. That is also happening in django 2.2

[Hristo Trendafilov]

Some of the images

[Hristo Trendafilov]

Other images

[Mariusz Felisiak]

Thanks for the report, however Paginator works for me. I'm also not sure why you're using _get_page() which is a private undocumented API. I don't think you've explained the issue in enough detail to confirm a bug in Django. Please reopen the ticket if you can debug your issue and provide details about why and where Django is at fault. If you're having trouble in debugging, see TicketClosingReasons/UseSupportChannels for ways to get help.

[Hristo Trendafilov]

Replying to Mariusz Felisiak:

Thanks for the report, however Paginator works for me. I'm also not sure why you're using _get_page() which is a private undocumented API. I don't think you've explained the issue in enough detail to confirm a bug in Django. Please reopen the ticket if you can debug your issue and provide details about why and where Django is at fault. If you're having trouble in debugging, see TicketClosingReasons/UseSupportChannels for ways to get help.