Django: Change Queryset of already defined Prefetch object - django

Question

Context
I really want to, but I don't understand how I can limit an already existing Prefetch object
Models
class MyUser(AbstractUser):
pass
class Absence(Model):
employee = ForeignKey(MyUser, related_name='absences', on_delete=PROTECT)
start_date = DateField()
end_date = DateField()
View
class UserAbsencesListAPIView(ListAPIView):
queryset = MyUser.objects.order_by('first_name')
serializer_class = serializers.UserWithAbsencesSerializer
filterset_class = filters.UserAbsencesFilterSet
Filter
class UserAbsencesFilterSet(FilterSet):
first_name = CharFilter(lookup_expr='icontains', field_name='first_name')
from_ = DateFilter(method='filter_from', distinct=True)
to = DateFilter(method='filter_to', distinct=True)
What do I need
With the Request there are two arguments from_ and to. I should return Users with their Absences, which (Absences) are bounded by from_ and/or to intervals. It's very simple for a single argument, i can limit the set using Prefetch object:
def filter_from(self, queryset, name, value):
return queryset.prefetch_related(
Prefetch(
'absences',
Absence.objects.filter(Q(start_date__gte=value) | Q(start_date__lte=value, end_date__gte=value)),
)
)
Similarly for to.
But what if I want to get a limit by two arguments at once?
When the from_ attribute is requested - 'filter_from' method is executed; for the to argument, another method filter_to is executed.
I can't use prefetch_related twice, I get an exception ValueError: 'absences' lookup was already seen with a different queryset. You may need to adjust the ordering of your lookups..
I've tried using to_attr, but it looks like I can't access it in an un-evaluated queryset.
I know that I can find the first defined Prefetch in the _prefetch_related_lookups attribute of queryset, but is there any way to apply an additional filter to it or replace it with another Prefetch object so that I can end up with a query similar to:
queryset.prefetch_related(
Prefetch(
'absences',
Absence.objects.filter(
Q(Q(start_date__gte=from_) | Q(start_date__lte=from_, end_date__gte=from_))
& Q(Q(end_date__lte=to) | Q(start_date__lte=to, end_date__gte=to))
),
)
)

Answer 1

django-filter seems to have its own built-in filter for range queries:
More info here and here
So probably just easier to use that instead:
def filter_date_range(self, queryset, name, value):
if self.lookup_expr = "range":
#return queryset with specific prefetch
if self.lookup_expr = "lte":
#return queryset with specific prefetch
if self.lookup_expr = "gte":
#return queryset with specific prefetch
I haven't tested this and you may need to play around with the unpacking of value but it should get you most of the way there.