#36342: Slicing a QuerySet with a result cache results in a list?
-------------------------------------+-------------------------------------
Reporter: Willem Van Onsem | Type:
| Cleanup/optimization
Status: new | Component: Database
| layer (models, ORM)
Version: 5.2 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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As
[https://github.com/django/django/blob/adf2991d32c24f8b2e549a25a7eda52f317a91a6/django/db/models/query.py#L401-L436
per code] if you slice a QuerySet with a result cache, we return the
sliced result cache, this thus means that for a `QuerySet`:
{{{
from django.contrib.auth.model import User
qs = User.objects.all()
bool(qs) # enable/disable
qs[:3].values('id')
}}}
will raise an error because `qs[:3]` returns a list, whereas if we comment
out the `bool`, it will still work.
This is done probably because of performance reasons: if we have a
`QuerySet`, and we already know the results, we can just work with these
results.
But I'm wondering if we "can have the cake and eat it too". We could for
example create a sliced copy of the queryset, and populate the result
cache of the queryset. Something along the lines of:
{{{
def __getitem__(self, k):
"""Retrieve an item or slice from the set of results."""
if not isinstance(k, (int, slice)):
raise TypeError(
"QuerySet indices must be integers or slices, not %s."
% type(k).__name__
)
if (isinstance(k, int) and k < 0) or (
isinstance(k, slice)
and (
(k.start is not None and k.start < 0)
or (k.stop is not None and k.stop < 0)
)
):
raise ValueError("Negative indexing is not supported.")
# remove below
# if self._result_cache is not None:
# return self._result_cache[k]
if isinstance(k, slice):
qs = self._chain()
if k.start is not None:
start = int(k.start)
else:
start = None
if k.stop is not None:
stop = int(k.stop)
else:
stop = None
qs.query.set_limits(start, stop)
if self._result_cache is not None:
# populate the QuerySet
qs._result_cache = self._result_cache[k]
return list(qs)[:: k.step] if k.step else qs
}}}
this thus means that, (a) unless we use a step, we always get a `QuerySet`
for slicing (since it is not always known in advance *if* the `QuerySet`
has a result cache, that can be the result of complicated code flows); and
(b) if the result cache was present, the queryset we generate will not
have to fetch the data, if we don't make more `QuerySet` calls.
--
Ticket URL: <https://code.djangoproject.com/ticket/36342>
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