id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
27228	prefetch_related with select_related takes a lot of query	aki33524	nobody	"mdoels.py
{{{#!python
class A(models.Model):
    pass


class B(models.Model):
    pass


class C(models.Model):
    a = models.ForeignKey(A)
    b = models.ForeignKey(B)
}}}

python manage.py shell
{{{#!python
for i in range(100):
    a = A()
    a.save()
    b = B()
    b.save()
    C(a=a, b=b).save()


queryset = A.objects.prefetch_related('c_set')
for v in queryset:
    print(v.c_set.count())
# 2 queries

queryset = C.objects.select_related()
for v in queryset:
    print(v.a.id)
# 2 queries

l = A.objects.prefetch_related(Prefetch('c_set', queryset=C.objects.select_related('b')))

for v in l:
    print(v.c_set.filter(b__id=1))
# 100 queries
}}}

I run this code in Django1.10.1, but the last takes a lot of queries. I think that the last can take only few queries."	Bug	closed	Database layer (models, ORM)	1.10	Normal	invalid	prefetch_related select_related		Unreviewed	0	0	0	0	0	0