#25496: ModelChoiceField generates extra queries by not respecting
prefetch_related
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Reporter: mdentremont | Owner: nobody
Type: Uncategorized | Status: closed
Component: Forms | Version:
Severity: Normal | Resolution: invalid
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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Changes (by charettes):
* status: new => closed
* needs_better_patch: => 0
* resolution: => invalid
* needs_tests: => 0
* needs_docs: => 0
Comment:
Hi mdentremont,
I think you misunderstood how `prefetch_related` and `count()` interact.
The `count()` method never takes prefetched data into consideration and
always result in a new query.
If you want to avoid an extra query I suggest you `annotate()` your
queryset with its count of related objects and adapt your models
`__unicode__` to lookup the annotated attribute first and fallback to
calling `self.related.count()`:
{{{#!python
from django.db import models
class RelatedObj(models.Model):
name = models.CharField(max_length=255)
class ObjWithRelated(models.Model):
related = models.ManyToManyField(RelatedObj)
def __unicode__(self):
related_count = getattr(self, 'related_count', None)
if related_count is None:
related_count = self.related.count()
return '{}: {}'.format(self.name, related_count)
queryset =
ObjWithRelated.objects.annotate(related_count=models.Count('related'))
field = ModelChoiceField(queryset)
}}}
Please use support channels before filling a ticket
wiki:TicketClosingReasons/UseSupportChannels
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Ticket URL: <https://code.djangoproject.com/ticket/25496#comment:1>
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