#33382: Different count and len result for a distinct QuerySet
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Reporter: 826541814 | Owner: nobody
Type: Uncategorized | Status: new
Component: Uncategorized | Version: 3.2
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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The problem arises when I use the len method on a distinct queryset. The
object obtained by the len method seems to be not distinct , But the query
did not change during this time.
I found what looks like the same Ticket ，But I'm not sure it's the same
reason [https://code.djangoproject.com/ticket/30655]
Here is a simple example that shows the contrary.
Models:
{{{
class Exam(models.Model):
paper = models.ForeignKey(ExamPaper, related_name='paper_exam',
on_delete=models.CASCADE, blank=True, null=True)
class StudentPaper(models.Model):
user = models.ForeignKey(User, related_name='user_exam_paper',
on_delete=models.CASCADE)
exam = models.ForeignKey(Exam, related_name='exam_student_paper',
on_delete=models.CASCADE)
}}}
Shell Output:
{{{
>>> from ExamManage.models import*
>>> exam = Exam.objects.first()
>>> exam.exam_student_paper.filter(is_pass=True).values('user_id').count()
521
>>> support_pass_userids =
exam.exam_student_paper.filter(is_pass=True).values('user_id').distinct()
>>> support_pass_userids.count()
484
>>> print(support_pass_userids.query)
SELECT DISTINCT `ExamManage_studentpaper`.`user_id`,
`ExamManage_studentpaper`.`id` FROM `ExamManage_studentpaper` WHERE
(`ExamManage_studentpaper`.`exam_id` = 5 AND
`ExamManage_studentpaper`.`is_pass`) ORDER BY
`ExamManage_studentpaper`.`id` DESC
>>> len(support_pass_userids)
521
>>> support_pass_userids.count()
521
>>> print(support_pass_userids.query)
SELECT DISTINCT `ExamManage_studentpaper`.`user_id`,
`ExamManage_studentpaper`.`id` FROM `ExamManage_studentpaper` WHERE
(`ExamManage_studentpaper`.`exam_id` = 5 AND
`ExamManage_studentpaper`.`is_pass`) ORDER BY
`ExamManage_studentpaper`.`id` DESC
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/33382>
Django <https://code.djangoproject.com/>
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