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
24873	Error while using Prefech on more than two levels	Gagaro	nobody	"Given the following models:


{{{
class A(models.Model):
    pass


class B(models.Model):
    a = models.ForeignKey(A, related_name='b')


class C(models.Model):
    b = models.ForeignKey(B, related_name='c')


class D(models.Model):
    c = models.ForeignKey(C, related_name='d')
}}}

And the following tests:

{{{
class PrefetchTest(TestCase):
    def setUp(self):
        a = A.objects.create()
        b = B.objects.create(a=a)
        c = C.objects.create(b=b)
        D.objects.create(c=c)

    def test_with_prefetch(self):
        c = C.objects.prefetch_related(Prefetch('d'))
        b = B.objects.prefetch_related(Prefetch('c', queryset=c))
        a = A.objects.prefetch_related(Prefetch('b', queryset=b))
        list(a)

    def test_without_prefetch(self):
        c = C.objects.prefetch_related('d')
        b = B.objects.prefetch_related(Prefetch('c', queryset=c))
        a = A.objects.prefetch_related(Prefetch('b', queryset=b))
        list(a)
}}}

The first test will fail while the second will succeed. The backtrace with Django 1.8 is:


{{{
Traceback (most recent call last):
  File ""/home/gagaro/django/modules/test/django_test/models_test/tests.py"", line 19, in test_with_prefetch
    list(a)
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 162, in __iter__
    self._fetch_all()
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 967, in _fetch_all
    self._prefetch_related_objects()
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 591, in _prefetch_related_objects
    prefetch_related_objects(self._result_cache, self._prefetch_related_lookups)
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 1516, in prefetch_related_objects
    obj_list, additional_lookups = prefetch_one_level(obj_list, prefetcher, lookup, level)
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 1615, in prefetch_one_level
    prefetcher.get_prefetch_queryset(instances, lookup.get_current_queryset(level)))
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/fields/related.py"", line 729, in get_prefetch_queryset
    for rel_obj in queryset:
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 162, in __iter__
    self._fetch_all()
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 967, in _fetch_all
    self._prefetch_related_objects()
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 591, in _prefetch_related_objects
    prefetch_related_objects(self._result_cache, self._prefetch_related_lookups)
  File ""/home/gagaro/.virtualenvs/django-test/local/lib/python2.7/site-packages/django/db/models/query.py"", line 1505, in prefetch_related_objects
    (through_attr, first_obj.__class__.__name__, lookup.prefetch_through))
AttributeError: Cannot find 'c' on C object, 'c__d' is an invalid parameter to prefetch_related()
}}}

I tested with Django 1.7, 1.8, 1.8.2 and master (commit 24718b7dccd64dfa118fe8136e7b175babec679b)."	Bug	new	Database layer (models, ORM)	dev	Normal		Prefetch prefetch_related		Unreviewed	0	0	0	0	0	0