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
34067	django.core.Paginator wrong query slicing	Hristo Trendafilov	nobody	"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 tried to add a breakpoint like so [[Image(https://imgur.com/SdaQUt6)]]
Then I get this result [[Image(https://imgur.com/5zzLNV0)]].

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 [[Image(https://imgur.com/yvn8KMO)]]
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 [[Image(https://imgur.com/d7J5BMZ)]] and that breakpoint is just run then the result is okay [[Image(https://imgur.com/ILDHAMN)]]

2. changed paginate_by to 10 

3. changed {{{order_by}}} in {{{get_queryset}}} to {{{-pk}}}

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


It is Django fault for those reasons:

* `order_by` might be complicated but is supported by the framework
* Chained `order_by` is also supported 
* queryset slicing is a feature of Django queryset API. 
* queryset slicing should act like list and string slicing and namely to provide you with results `[ from : to ]` indexes provided and not to change or lose data in between. Or in other words - what is passed in the `Paginator.object_list` should be just sliced `[ from : to ]`. `Paginator` should never care for the queryset, what is the ordering of the queryset, what is excluded and so on - it should just work with it.
* changing `paginate_by` to `10` or greater fixes the problem. `paginate_by` is a standard and documented Django ListView setting.
* changing `order_by` to some other `order_by` also fixes the problem. `order_by` should work the same or if any limitations those should be well documented 
* removing `order_by` also fixes the problem.

P.S. That is also happening in Django 2.2"	Bug	closed	Core (Other)	3.2	Normal	needsinfo	Paginator,slice,queryset		Unreviewed	0	0	0	0	0	0