Error 'NoneType' object has no attribute '_default_manager' when I create a view of Model instance

Question

I'm new in Django and I'm developing an app but I'm stuck in this Error. First I want to apologize for my English since is not my first language and then I hope I'm in the right place to ask for little help.

I'm developing a website about Publication of articles. The page "Publication" return a list of the model instances - in my case all the publications in the db.

What I'm trying to do is to assign a url to each of the instances of my Model, such that the user can click on it and see the page which is populated with other relevant information of that instance.

To do this I'm using get_absolute_url in my template and DetailView in my view. The list of all the instance works fine but when I click on each of the instance I run the error 'NoneType' object has no attribute '_default_manager' . I google already and try to follow some guidelines here and there, and try to find solution in Django doc couldnt figure out the solution...

Here my code:

models.py

class Publications(MPTTModel):
    code = models.CharField(max_length=50)
    title = models.CharField(max_length=150)
    date = models.DateField(null=True)
    magazine = models.CharField(max_length=50)
    country = models.CharField(max_length=30)
    slug = models.SlugField(max_length=150)
    img01 = models.ImageField(upload_to="photo", blank=True, null=True)
    link = models.URLField(max_length=200, blank=True, null=True)
    template = models.ForeignKey("Template", related_name="publications", null=True, blank=True)
    parent = models.ForeignKey('self', null=True, blank=True, related_name='children')
    control_field = models.CharField(max_length=15)
    class Meta:
        verbose_name_plural = "Publications"
    def __unicode__(self):
        return self.title
    def get_absolute_url(self):
        return reverse('mag-publication',args=(self.slug,))

publications.html

{% load mptt_tags %}
{% load sito_tags %}
{% full_tree_for_model myApp.Publications as publications %}
{% for publications,structure in publications|tree_info %}  
    {% if publications.img01 %}
    <div id="title_publication_container">
        <a href="{{ publications.get_absolute_url }}">{{ publications.magazine }}</a>
    </div>                      
    {% else %}                          
        ....    
    {% endif %} 
{% endfor %}

urls.py

urlpatterns = patterns('',  
url(r'^(?P<id_page>\d+)/(?P<slug>[\w-]+)/$', pages,),
url(r'^(?P<id_page>\d+)/(?P<slug_page>[\w-]+)/(?P<id>\d+)/(?P<slug>[\w-]+)/$', projects,),
url(r'^mag-publication/(?P<slug>[\w-]+)/$', PublicationDetailView.as_view() , name='mag-publication'),

)

view.py

class PublicationView(object):
    queryset = Publications.objects.all()

class PublicationListView(PublicationView, ListView):
    paginate_by = 20

class PublicationDetailView(PublicationView, DetailView):
    slug_field = 'slug'

The error show me this Debug informations:

AttributeError at /6/publications/*title_publication*/
**'NoneType' object has no attribute '_default_manager'**

Request Method:     GET
Request URL:    .../6/publications/*title_publication*/
Django Version:     1.5.4
Exception Type:     AttributeError
Exception Value:    'NoneType' object has no attribute '_default_manager'
Exception Location:     /home/gabriele/virtual_envs/virt2/lib/python2.7/site-packages/feincms/views/cbv/views.py in get_object, line 20
Python Executable:  /home/gabriele/virtual_envs/virt2/bin/python

Maybe the problem has something to do with feincms as it said in the "Exception Location" but everything came out after I tried to work on the instance of the Model.

Thank you for your help.

Answer 1

You probably didn't add feincms.module.page to your INSTALLED_APPS as per the documentation. If you follow the traceback, the error appears in get_object() where it tries to access the page model.

Are you using an older FeinCMS version? Newer versions raise a warning in that case.