Django Inheritance and Permalinks

Question

I'm creating a simple CMS in django, with multiple "modules" (each as a django app). I've set up the following models:

class FooObject(models.Model):
    id = models.SlugField(primary_key=True)
    name = models.CharField(max_length=255)
    creator = models.ForeignKey(auth.models.User, editable=False, related_name="createdby")

class FooPage(FooObject):
    content = models.TextField(blank=True, null=True)

    @models.permalink
    def get_absolute_url(self):
        return ('page', (), {'page_id':self.id}

class FooSubitem(FooObject):
    parent = models.ForeignKey(FooPage, related_name='subitems')

In each of the modules, I create a subclass of FooPage, and at least one subclass of FooSubitem, e.g.

# in FooBlog.models
class FooBlog(FooPage):
    owner = models.ForeignKey(auth.models.User, editable=False)

    @models.permalink
    def get_absolute_url(self):
        return ('blog', (), {'blog_id':self.id})

class FooPost(FooSubitem):
    post_time = models.DateTimeField(auto_now_add=True)

and

# in FooGallery.models
class FooGallery(FooPage):
    location = models.CharField(max_length=255)

    @models.permalink
    def get_absolute_url(self):
        return ('gallery', (), {'gallery_id':self.id})

class FooImage(FooSubitem):
    image_file = models.ImageField(upload_to='foogallery')

These are simplifications, but should give you a good idea of what I'm trying to do. In the admins for FooPost and FooImage, I restrict the parent selection list to their corresponding parent pages.

My problem arises when I try to use these in a template. In each view, I have the following:

page_list = FooPage.objects.all()

which returns a list of all FooPages, of both FooBlog and FooGallery types. However, when I iterate through this list:

{% for page in page_list %}{{ page.get_absolute_url }}{% endfor %}

it returns the 'page' url pattern, not the 'blog' or 'gallery' url pattern.

How do I make this work without having to rewrite the code when I want to add a FooCalendar module later on? I want to make sure this works with any possible module.

Thanks,

• Lexo

Answer 1

The classic solution to this problem tends to be adding a ContentType to the superclass which stores the type of subclass for that instance. This way you can rely on a consistent API that returns the related subclass object of the appropriate type.

Answer 2

You can avoid adding a content type field by using the InheritanceManager from django-model-utils.

Then, if you call .select_subclasses on a queryset, it will downcast all of the objects, for example:

FooPage.objects.select_subclasses().all()

Answer 3

FooPage.objects.all() returns all the objects of type FooPage, these objects will be mix of underlying db table rows for FooPage, FooBlog, FooGallery. To get the correct URL you should get the FooBlog or FooGallery object e.g.

page.fooblog.get_absolute_url()

it may throw FooBlog.DoesNotExist error if page is simply a page object i.e created via FooPage, so to get correct urls you may do something like this

   urls = []
   for page in FooPage.objects.all():
        try:
            page = page.fooblog
        except FooBlog.DoesNotExist:
            pass

            urls.append(page.get_absolute_url())

alternatively you may try to make FooPage a abstractclass if you do not want FooPage to be a real table.