#22268: values_list() on a ManyToManyField returns extra None's when iterated
over.
-------------------------------------+-------------------------------------
Reporter: k_sze | Owner:
Type: Bug | anubhav9042
Component: Database layer | Status: assigned
(models, ORM) | Version: master
Severity: Normal | Resolution:
Keywords: orm, values_list, | Triage Stage: Accepted
ManyToManyField | Needs documentation: 0
Has patch: 0 | Patch needs improvement: 0
Needs tests: 0 | UI/UX: 0
Easy pickings: 0 |
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Comment (by k_sze):
Actually the whole behaviour of `values()` and `values_list()` seems to be
wrong once you mix M2M fields into the picture. I just tried creating more
Class and Student objects:
{{{
# New student, Joe Blo
>>> joe_blo = Student(surname='Blo', given_name='Joe')
>>> joe_blo.save()
# New class, Discrete Mathematics
>>> discrete_math = Class(name='Discrete Mathematics', school=concordia)
>>> discrete_math.save()
# Enroll both john_smith and joe_blo in discrete_math
discrete_math.students.add(joe_blo)
discrete_math.students.add(john_smith)
}}}
First, let's look at `values()` queries:
{{{
>>> Class.objects.values()
[{u'id': 1, 'name': u'Software Engineering', 'school_id': 1}, {u'id': 2,
'name': u'Computer Engineering', 'school_id': 1}, {u'id': 4, 'name':
u'Discrete Mathematics', 'school_id': 1}]
}}}
That's 3 dictionaries because we now have 3 Class objects. That seems
fine. On the other hand, why don't students appear here?
{{{
>>> Class.objects.values('name', 'students')
[{'students': 1, 'name': u'Software Engineering'}, {'students': None,
'name': u'Computer Engineering'}, {'students': 1, 'name': u'Discrete
Mathematics'}, {'students': 2, 'name': u'Discrete Mathematics'}]
}}}
That's 4 dictionaries even though we only have 3 Class objects (soen,
coen, and discrete_math). Can you imagine what would happen if the Class
object had more M2M fields? The number of dictionaries returned would
probably explode exponentially! Why aren't the students combined into a
tuple or a list? Shouldn't it be 3 dictionaries like this:
{{{
[
{'students': (1,), 'name': u'Software Engineering'},
{'students': (,), 'name': u'Computer Engineering'}, # Note the empty
tuple for students, instead of None
{'students': (1, 2), 'name': u'Discrete Mathematics'},
]
}}}
Now, let's look at `values_list()` queries:
{{{
>>> Class.objects.values_list()
[(1, 1, u'Software Engineering'), (2, 1, u'Computer Engineering'), (4, 1,
u'Discrete Mathematics')]
}}}
3 tuples. Again, this seems fine except for the fact that students aren't
included.
{{{
>>> Class.objects.values_list('name', 'students')
[(u'Software Engineering', 1), (u'Computer Engineering', None),
(u'Discrete Mathematics', 1), (u'Discrete Mathematics', 2)]
}}}
4 tuples even though we have 3 Class objects only! Shouldn't this be 3
tuples like this:
{{{
[
(u'Software Engineering', (1,)),
(u'Computer Engineering', (,)), # Note the empty tuple instead of None
(u'Discrete Mathematics', (1, 2)),
]
}}}
--
Ticket URL: <https://code.djangoproject.com/ticket/22268#comment:15>
Django <https://code.djangoproject.com/>
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