#24873: Error while using Prefech on more than two levels
-------------------------------------+-------------------------------------
Reporter: Gagaro | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: master
(models, ORM) | Keywords: Prefetch
Severity: Normal | prefetch_related
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
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).
--
Ticket URL: <https://code.djangoproject.com/ticket/24873>
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