#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):
To demonstrate how bad things become once you have multiple M2M fields, I
created a new `Language` model and add it as a M2M field in the Class
model:
{{{
from django.db import models
# other Model definitions skipped ...
class Class(models.Model):
school = models.ForeignKey(School)
name = models.CharField(unique=True, max_length=255)
students = models.ManyToManyField('Student')
available_in_languages = models.ManyToManyField('Language')
class Language(models.Model):
name = models.CharField(max_length=255)
}}}
Now I create new languages and add them to the Discrete Mathematics class
(which now has 2 students, if you recall):
{{{
# French
>>> french = Language(name='French')
>>> french.save()
# English
>>> english = Language(name='English')
>>> english.save()
# Add them to Discrete Mathematics class
discrete_math = Class.objects.get(name='Discrete Mathematics')
discrete_math.available_in_languages.add(french)
discrete_math.available_in_languages.add(english)
}}}
Now I make a `values()` query:
{{{
>>> Class.objects.values('available_in_languages', 'students')
[{'students': 1, 'available_in_languages': None}, {'students': None,
'available_in_languages': None}, {'students': 1, 'available_in_languages':
2}, {'students': 2, 'available_in_languages': 2}, {'students': 1,
'available_in_languages': 1}, {'students': 2, 'available_in_languages':
1}]
}}}
That's 6 dictionaries now!
Similarly, with `values_list()`:
{{{
>>> Class.objects.values_list('available_in_languages', 'students')
[(None, 1), (None, None), (2, 1), (2, 2), (1, 1), (1, 2)]
}}}
6 tuples even though we only have 3 Class objects.
--
Ticket URL: <https://code.djangoproject.com/ticket/22268#comment:16>
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