February 2024
Django doesn't allow negative indexes on QuerySets, resulting in an exception if you try. Eg if you try to get the last item of a queryset with the following you'll get:
Foo.objects.all()[-1]
… traceback …
ValueError: Negative indexing is not supported.
So how does last() work?
The methods first() and last() force ordering on the queryset to produce deterministic results, if not already
ordered, with last() behaving similarly to first() except with the ordering reversed.
Borrowing from last() we can extend __getitem__() to do the same thing for negative indexes then invert the index
(and subtract 1 to account for the 0 base):
class OurQuerySet(QuerySet):
def __getitem__(self, key):
if isinstance(key, int) and key < 0:
if self.ordered:
queryset = self.reverse()
else:
self._check_ordering_first_last_queryset_aggregation(method="last")
queryset = self.order_by("-pk")
key = abs(key) - 1
return queryset[key]
else:
return super().__getitem__(key)