lookup was already seen with a different queryset

Question

I sort the list of credits by ajax request. The first time everything goes well, I get sorted list of credits and update the cache. But when I try to sort again, I get an error.

'creditpayments' lookup was already seen with a different queryset. You may need to adjust the ordering of your lookups.

If the cache is not to update, then there will be no error. But I need to update it...

models

class Credit(models.Model):
    pass


class CreditPayment(models.Model):
    credit = models.ForeignKey(Credit, on_delete=models.CASCADE, related_name='creditpayments')
    rate = models.DecimalField(_('rate'), max_digits=7, decimal_places=2)

views

class SortedCreditsList(ListView):
    def get(self, *args, **kwargs):
        if self.request.is_ajax():
            credits = cache.get('credits')
            prefetch = Prefetch('creditpayments', CreditPayment.objects.all())
            credits = credits.prefetch_related(prefetch).annotate(min_rate=Min('creditpayments__rate'))
            credits = credits.order_by('min_rate')
            cache.set('credits', credits)
            credits = credits.filter(best=False, hot=False)

            template_ajax = render_to_string(
                    template_name='credits/includes/credits_filter_result.html',
                    context={
                        'credits': credits,
                    })

            return JsonResponse({'success': True, 'template': template_ajax})

Traceback

ValueError at /sorted_kredit/
'creditpayments' lookup was already seen with a different queryset. You may need to adjust the ordering of your lookups.

Traceback:  

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  149.                     response = self.process_exception_by_middleware(e, request)

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/core/handlers/base.py" in get_response
  147.                     response = wrapped_callback(request, *callback_args, **callback_kwargs)

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/views/generic/base.py" in view
  68.             return self.dispatch(request, *args, **kwargs)

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/views/generic/base.py" in dispatch
  88.         return handler(request, *args, **kwargs)

File "/home/m0nte-cr1st0/work_projects/startup/finbee/credits/views.py" in get
  555.             cache.set('credits', credits)

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/core/cache/backends/locmem.py" in set
  75.         pickled = pickle.dumps(value, pickle.HIGHEST_PROTOCOL)

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/db/models/query.py" in __getstate__
  211.         self._fetch_all()

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/db/models/query.py" in _fetch_all
  1076.             self._prefetch_related_objects()

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/db/models/query.py" in _prefetch_related_objects
  656.         prefetch_related_objects(self._result_cache, self._prefetch_related_lookups)

File "/home/m0nte-cr1st0/.virtualenvs/finbee/local/lib/python2.7/site-packages/django/db/models/query.py" in prefetch_related_objects
  1397.                                  "You may need to adjust the ordering of your lookups." % lookup.prefetch_to)

Answer 1

The error caused by prefetching model that you already prefetched.

When you get credits value from cache, the object already prefetched.

if hasattr(cache, 'credits'): # Already Cached
    credits = cache.get('credits')

else:
    prefetch = Prefetch('creditpayments', CreditPayment.objects.all())
    credits = credits.prefetch_related(prefetch).annotate(min_rate=Min('creditpayments__rate'))
    credits = credits.order_by('min_rate')
    cache.set('credits', credits)

I think this code makes your code works properly.