Django AttributeError: 'str' object has no attribute '_default_manager'

Question

The following error seems to occur randomly on my live server (i.e. through apache mod_wsgi) but never in development (i.e. localhost python manage.py runserver).

Note this happens infrequently and is not something that can be reproduced easily or each time a specific url is accessed.

I have seen various answers posted both here on SO and on google but there does not seem to be any definitive reason as to why this error occurs. Maybe this is because the error is fairly generic but the most common answer seems to be due to circular import errors. Another answer I've seen is that model FK field references have not been the correct case (e.g. applabel.model instead of applabel.Model) but all my model FK fields are correct.

The cause of the error seems to point to one of my admin.py files. This file did originally import custom form classes from a forms.py file. Both the admin.py file and forms.py file imported the same models from a models.py file. Therefore I moved the form classes to the admin.py file in case there was a circular reference occurring here but I still occasionally get these errors.

Could anyone shed any light as to why this error occurs and why so randomly? I always ensure the relevant services are restarted after a code update.

Traceback is:

Traceback (most recent call last):

File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 101, in get_response
request.path_info)

File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py", line 250, in resolve
for pattern in self.url_patterns:

File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py", line 279, in _get_url_patterns
patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)

File "/usr/local/lib/python2.7/dist-packages/django/core/urlresolvers.py", line 274, in _get_urlconf_module
self._urlconf_module = import_module(self.urlconf_name)

File "/usr/local/lib/python2.7/dist-packages/django/utils/importlib.py", line 35, in import_module
__import__(name)

File "/myproject/urls.py", line 6, in <module>
admin.autodiscover()

File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/__init__.py", line 26, in autodiscover
import_module('%s.admin' % app)

File "/usr/local/lib/python2.7/dist-packages/django/utils/importlib.py", line 35, in import_module
__import__(name)

File "/myproject/myapps/app/admin.py", line 61, in <module>
class CardAdminForm(forms.ModelForm):

File "/usr/local/lib/python2.7/dist-packages/django/forms/models.py", line 205, in __new__
opts.exclude, opts.widgets, formfield_callback)

File "/usr/local/lib/python2.7/dist-packages/django/forms/models.py", line 159, in fields_for_model
formfield = f.formfield(**kwargs)

File "/usr/local/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 913, in formfield
'queryset': self.rel.to._default_manager.using(db).complex_filter(self.rel.limit_choices_to),

AttributeError: 'str' object has no attribute '_default_manager'

Packages and versions

• Warning: cannot find svn location for pymssql==2.0.0b1-dev-20111019
• Warning: cannot find svn location for distribute==0.6.24dev-r0
• Django==1.3.3
• GnuPGInterface==0.3.2
• Landscape-Client==12.05
• PAM==0.4.2
• PIL==1.1.7
• Twisted-Core==11.1.0
• apt-xapian-index==0.44
• argparse==1.2.1
• chardet==2.0.1
• command-not-found==0.2.44
• ## FIXME: could not find svn URL in dependency_links for this package: distribute==0.6.24dev-r0
• django-debug-toolbar==0.9.4
• django-rosetta==0.6.8
• httplib2==0.7.2
• iotop==0.4.4
• keyring==0.7.1
• language-selector==0.1
• launchpadlib==1.9.12
• lazr.restfulclient==0.12.0
• lazr.uri==1.0.3
• mercurial==2.0.2
• oauth==1.0.1
• psycopg2==2.4.5
• pyOpenSSL==0.12
• pycrypto==2.4.1
• ## FIXME: could not find svn URL in dependency_links for this package:pymssql==2.0.0b1-dev-20111019
• pyserial==2.5
• python-apt==0.8.3ubuntu7
• python-debian==0.1.21ubuntu1
• reportlab==2.5
• simplejson==2.3.2
• ufw==0.31.1-1
• wadllib==1.3.0
• wsgiref==0.1.2
• xlwt==0.7.4
• zope.interface==3.6.1

Database: Postgresql 9.1.5

CardAdmin and CardAdminForm:

class CardAdmin(admin.ModelAdmin):
    form = CardAdminForm
    raw_id_fields = ('cust', 'acc', 'vehicle', 'driver')
    list_display = ('id', 'pan', 'name', 'expiry', 'created', 'modified')
    list_filter = ('status', )
    search_fields = ['id', 'pan']
admin.site.register(Card, CardAdmin)

class CardAdminForm(forms.ModelForm):
    """
    A Form for Cards (Admin console)
    """

    def __init__(self, *args, **kwargs):
        super(CardAdminForm, self).__init__(*args, **kwargs)
        self.fields['cust'].required = True
        self.fields['acc'].required = True
        self.fields['name'].required = True
        self.fields['code'].widget = forms.PasswordInput()
        self.fields['code'].max_length = 6

    class Meta:
        model = Card
        fields = (
            'cust',
            'name',
            'acc',
            'no',
            'code',
            'type',
            'status',
            'address_1',
            'address_2',
            'zip',
            'city',
            'country',
            'phone_no',
            'expiry',
            'vehicle',
            'driver'
        )

    def save(self, commit=True):
        # Save some additional data.
        form_instance = super(CardAdminForm, self).save(commit=False)

        cleaned_data = self.cleaned_data
        form_instance.pan = '%s%s%s'\
        % (
            cleaned_data['acc'].iso.number,
            cleaned_data['acc'].number,
            cleaned_data['no']
        )

        if commit:
            form_instance.save()
        return form_instance

Answer 1

Quick note for people still finding this old issue: This case can also be caused by a ForeignKey/ManyToMany/OnetoOne that uses a string as reference that is invalid (eg: not correctly pointing to a model).

I was updating/refactoring a project and ran into this. Turned out it was just a typo.

Kinda weird django doesn't notify clearly it cannot resolve the string, could be because other apps confused it.