#18510: Options._fill_related_objects_cache should assert for app_cache_ready()
or
deal with app_cache_ready() == False
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Reporter: vanschelven | Owner: nobody
Type: Uncategorized | Status: new
Component: Database layer (models, ORM) | Version: 1.4
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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It is possible to end up in Options._fill_related_many_to_many_cache
(https://github.com/django/django/blob/1.4/django/db/models/options.py#L388)
while app_cache_ready() is False. Since that method iterates over
get_models() (line 401) and saves its result in an object attribute
regardless of the value of app_cache_ready() in line 409 this may result
in the _related_objects_cache missing elements, causing get_field_by_name
(and possibly other methods) to fail.
Steps to reproduce:
{{{
# models.py
from django.db import models
from django.contrib.auth.models import User
User._meta.get_field_by_name('username')
# alternatively, import the below to trigger the error.
# from django.contrib.auth.admin import UserAdmin
class UserProfile(models.Model):
user = models.ForeignKey(User, unique=True, editable=False)
first_name = models.CharField(max_length=255, blank=True)
User._meta.ordering = ('userprofile__first_name',)
}}}
{{{
# tests.py
from django.test import TestCase
from django.contrib.auth.models import User
class SimpleTest(TestCase):
def test_get_field_by_name(self):
# no assertions needed, the test simply fails
User.objects.create(username='test')
User._meta.get_field_by_name('userprofile')
# similarly methods using get_field_by_name themselves fail, such
as
u = User.objects.all()[0]
}}}
{{{
----------------------------------------------------------------------
Traceback (most recent call last):
File "/tmp/ve13/reverselookup/userprofile/tests.py", line 9, in
test_get_field_by_name
User._meta.get_field_by_name('userprofile')
File "/tmp/ve14/lib/python2.6/site-
packages/django/db/models/options.py", line 315, in get_field_by_name
% (self.object_name, name))
!FieldDoesNotExist: User has no field named 'userprofile'
}}}
This fails in both Django 1.3 and Django 1.4.
Solution:
The canonical solution in options.py seems to be to check for the value of
app_cache_ready() before actually setting the internal cache attribute.
E.g.
https://github.com/django/django/blob/1.4/django/db/models/options.py#L447
. I'm not sure whether that would fix all problems caused by this or
whether an assert for app_cache_ready() at the top of the method would be
more appropriate.
I'm currently not able to come up with a proper way of providing tests for
the situation above, because the global state of app loading and such
makes coming up with tests somewhat harder. If anyone can help me with
that, that would be great. Alternatively perhaps someone is willing to
take the above method of reproduction and the fact that no old tests break
as enough reason for a patch?
Admittedly the above scenario appears contrived (calling
User._meta.get_field_by_name('username') in a models.py file). However,
this is simply the easiest way to reproduce the problem.
I ran into this when further debugging #18507 . In that case I was using
from django.contrib.auth.admin import !UserAdmin somewhere early in one of
my models files. Since Django 1.3 adds more validation to the admins, one
of which contained a call to get_field_by_name this resulted in a rather
hard-to-debug problem when upgrading from Django 1.2 to Django 1.3 (or
1.4).
--
Ticket URL: <https://code.djangoproject.com/ticket/18510>
Django <https://code.djangoproject.com/>
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