sitemaps crashes with "isinstance() arg 2 must be a class, type, or tuple of classes and types"

[Chen Xu]

I am using Django version 1.9.8, and mezzanine version 4.1.0.
The following code failed:

from django.contrib.sitemaps.views import sitemap
from mezzanine.core.sitemaps import DisplayableSitemap
sitemaps = {"sitemaps": {"displayable": DisplayableSitemap}}
urlpatterns += [url("^sitemap\.xml$", sitemap, sitemaps)]

The trace back:

TypeError at /sitemap.xml
isinstance() arg 2 must be a class, type, or tuple of classes and types
Request Method:	GET
Request URL:	https://xxxxxxxxxx.com:8787/sitemap.xml
Django Version:	1.9.8
Exception Type:	TypeError
Exception Value:	
isinstance() arg 2 must be a class, type, or tuple of classes and types
Exception Location:	/srv/xxxxxxxxxx/virtualenvs/corpsite/local/lib/python2.7/site-packages/django/db/models/fields/related.py in get_default, line 908
Python Executable:	/srv/xxxxxxxxxx/virtualenvs/corpsite/bin/uwsgi
Python Version:	2.7.6
Python Path:	
['.',
 '',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/src/supervisor',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7/plat-x86_64-linux-gnu',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7/lib-tk',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7/lib-old',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/lib/python2.7/lib-dynload',
 '/usr/lib/python2.7',
 '/usr/lib/python2.7/plat-x86_64-linux-gnu',
 '/usr/lib/python2.7/lib-tk',
 '/srv/xxxxxxxxxx/virtualenvs/corpsite/local/lib/python2.7/site-packages',
 '/srv/xxxxxxxxxx/www/private']
Server time:	Fri, 22 Jul 2016 17:48:22 -0700
...
...

/srv/xxxxxxxxxx/virtualenvs/corpsite/local/lib/python2.7/site-packages/django/db/models/fields/related.py in get_default
        if isinstance(field_default, self.remote_field.model): ...
▶ Local vars
Variable	Value
self: <django.db.models.fields.related.ForeignKey: site>
field_default: None

The problem is here: self.remote_field.model is not a type of class, but unicode.

[Tim Graham]

Seems to be a regression in 1.9 (the sample code works in 1.8) although I've having trouble bisecting it due to other crashes in mezzanine at various points in Django's history. If you could create a minimal project that reproduces the problem without mezzazine, that would be ideal as we'll need a regression test anyway.

[Chen Xu]

I found out this is a known bug in mezzanine, not a django bug. We can close it here.