#22550: QuerySet last() and reverse() confusing when used with ordered slices
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Reporter: jdufresne | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.6
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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I ran all tests below with MySQL backend.
Given the following model:
{{{
class MyModel(models.Model):
name = models.CharField(max_length=100)
class Meta:
ordering = ['name']
def __unicode__(self):
return self.name
}}}
From the following test script I receive the output:
{{{
import os
os.environ.setdefault("DJANGO_SETTINGS_MODULE", "djtest.settings")
from myapp.models import MyModel
MyModel.objects.all().delete()
for i in range(10):
m = MyModel(name='model%d' % i)
m.save()
print MyModel.objects.all()[0:5]
print MyModel.objects.all()[0:5].last()
print MyModel.objects.all()[0:5].reverse()
}}}
{{{
[<MyModel: model0>, <MyModel: model1>, <MyModel: model2>, <MyModel:
model3>, <MyModel: model4>]
model9
[<MyModel: model9>, <MyModel: model8>, <MyModel: model7>, <MyModel:
model6>, <MyModel: model5>, <MyModel: model4>, <MyModel: model3>,
<MyModel: model2>, <MyModel: model1>, <MyModel: model0>]
}}}
In my opinion, the lines that show the output of {{{last()}}} and
{{{reverse()}}} are confusing. My naive expectation was that each of these
would be operating on the slice. That is, the last value from the slice or
the reverse order of the slice.
{{{
model4
[<MyModel: model4>, <MyModel: model3>, ...
}}}
Of course, after thinking about how this might get translated to SQL I
understand why this happens. Making this a leaky abstraction. In my
opinion, if expected behavior can't be realized an exception preventing
the operation makes more sense.
The original reason I bumped into this is I was creating a custom
Paginator to create page labels based on the first and last item on the
page. As Paginator uses slices, calling {{{last()}}} produced unexpected
(to me) results.
--
Ticket URL: <https://code.djangoproject.com/ticket/22550>
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