#20143: Lazy loading of related fields does not work
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Reporter: andreas_pelme | Owner: nobody
Type: Bug | Status: new
Component: Database layer (models, ORM) | Version: 1.5
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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There is a problem with the lazy loading mechanism of related fields.
Steps to reproduce (reproduced with 1.4 and 1.5):
{{{
$ django-admin.py startproject lazy_model_loading
$ cd lazy_model_loading/
lazy_model_loading $ python manage.py startapp foo
lazy_model_loading $ python manage.py startapp bar
lazy_model_loading $ echo "class Foo(models.Model): bar =
models.ForeignKey('bar.Bar')" >> foo/models.py
lazy_model_loading $ echo "class Bar(models.Model): pass" >> bar/models.py
lazy_model_loading $ echo "INSTALLED_APPS = ['foo', 'bar']" >>
lazy_model_loading/settings.py
}}}
This is the expected behavior (this is the way everything should work)
{{{
lazy_model_loading $ python manage.py shell --plain
Python 2.7.2 (default, Jun 20 2012, 16:23:33)
[GCC 4.2.1 Compatible Apple Clang 4.0 (tags/Apple/clang-418.0.60)] on
darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from foo.models import Foo
>>> Foo()
<Foo: Foo object>
}}}
However, when running python directly, the issue is triggered:
{{{
lazy_model_loading $ DJANGO_SETTINGS_MODULE=lazy_model_loading.settings
python
Python 2.7.2 (default, Jun 20 2012, 16:23:33)
[GCC 4.2.1 Compatible Apple Clang 4.0 (tags/Apple/clang-418.0.60)] on
darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from foo.models import Foo
>>> Foo()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Users/andreas/.virtualenvs/removeme/lib/python2.7/site-
packages/django/db/models/base.py", line 397, in __init__
val = field.get_default()
File "/Users/andreas/.virtualenvs/removeme/lib/python2.7/site-
packages/django/db/models/fields/related.py", line 1038, in get_default
if isinstance(field_default, self.rel.to):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes
and types
}}}
The difference here is that manage.py shell calls get_models which forces
all models to load (which, judging by the comment is an old workaround for
another lazy load bug):
https://github.com/django/django/blob/master/django/core/management/commands/shell.py#L58
If get_models is removed from the shell management command - it will show
the same problem.
This is probably the same bug as described here:
http://stackoverflow.com/questions/5776047/django-modelform-giving-
isinstance-arg-2-must-be-a-class-type-or-tuple-of-cl
http://stackoverflow.com/questions/14386536/instantiating-django-model-
raises-typeerror-isinstance-arg-2-must-be-a-class (altough the accepted
answer does not seem to be 100 % correct - I am pretty sure it does this
bug does not depend on settings.DEBUG)
This bug can lead to very hard-to-debug problems since it depends on the
the implicit import order before the model being initiated. The exception
that is thrown is also not very helpful.
I would be happy to provide a fix, but I am not sure where to start fixing
this... Pointers would be appreciated.
--
Ticket URL: <https://code.djangoproject.com/ticket/20143>
Django <https://code.djangoproject.com/>
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