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
20143	Lazy loading of related fields does not work for non-loaded models	Andreas Pelme	nobody	"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."	Bug	closed	Database layer (models, ORM)	1.5	Normal	fixed	app-loading	djangoproject.com@…	Accepted	1	0	0	1	0	0