prefetch_related with select_related takes a lot of query

[aki33524]

mdoels.py

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

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.

[Tim Graham]

See the note in the ​prefetch_related() docs ("Remember that, as always with QuerySets, ...").