Cross posted from: stack
overflow<http://stackoverflow.com/questions/15923809/django-queryset-in-operator-on-two-querysets-fails-on-first-call>
.
When using the 'in' operator on two querysets, the first time the call is
made it fails.
from django.db import models
class Category(models.Model):
name = models.CharField(max_length=100)
slug = models.SlugField(max_length=100, db_index=True, unique=True)
def __unicode__(self):
return self.name
class Director(models.Model):
name = models.CharField(max_length=100)
slug = models.SlugField(max_length=100, db_index=True, unique=True)
def __unicode__(self):
return self.name
class Project(models.Model):
title = models.CharField(max_length=100)
slug = models.SlugField(max_length=100, db_index=True, unique=True)
director = models.ForeignKey(Director, related_name='projects_via_director')
categories = models.ManyToManyField(Category,
related_name='projects_via_category')
def __unicode__(self):
return self.title
category_list = Category.objects.prefetch_related('projects_via_category')
director_list = Director.objects.prefetch_related('projects_via_director')
other_cats = category_list.filter(
projects_via_category__director__slug='steven-spielberg')
print category_list # [<Category: Film>, <Category: Commercial>]print
other_cats # [<Category: Film>]print category_list[0] in other_cats #
Falseprint category_list[0] in other_cats # True
If I remove 'prefetch_related' and use 'all' instead then the problem is
gone, however this uses many more DB calls.
How can I get the correct result first time using prefetch_related?
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