Django model subclassing: Get the subclass by querying the superclass

Question

The following code is given:

class BaseMedium(models.Model):
    title = models.CharField(max_length=40)
    slug = models.SlugField()

class A(BaseMedium):
         


        

Answer 1

You should check the solution posted by Carl Meyer some time ago. It internally uses the ContentType approach, but it encapsulates it very elegantly .

He also points to an alternative, and more efficient solution, that doesn't need to store an aditional field on the database, but it will only work for direct child classes. If you have several inheritance levels, the first solution is better.

Answer 2

This worked for me (using self.subclass_name_in_lower_case): In this example subclasses are TextTreeItem, CategoryTreeItem and KeywordTreeItem.

class TreeItem(MPTTModel):
    parent = TreeForeignKey('self', on_delete=models.CASCADE, verbose_name=_('Parent'),
                            null=True, blank=True, related_name='%(class)s_related')

    objects = CustomTreeManager()

    @property
    def daughter(self):
        try:
            return self.texttreeitem
        except TreeItem.texttreeitem.RelatedObjectDoesNotExist:
            pass

        try:
            return self.categorytreeitem
        except TreeItem.categorytreeitem.RelatedObjectDoesNotExist:
            pass

        try:
            return self.keywordtreeitem
        except TreeItem.keywordtreeitem.RelatedObjectDoesNotExist:
            return self

Answer 3

The only way to do this is to explicitly store on the base model what type it is. So have a derived_type (or whatever) field on BaseMedium, and set it on save. Then you can have a get_derived_type method:

def get_derived_type(self):
    if self.derived_type ==  'A':
        return self.a
    elif self.derived_type == 'B':
        return self.b

and so on.

Answer 4

Thanks mr. Roseman for your reply. I developed your idea a bit further. Here is what I came up with:

def related_object(self, default_pointer_name='_ptr'):
        models = [A,B] #models
        object = None

        argument = '%s%s' %(self.__class__.__name__.lower(), default_pointer_name)
        query = { argument : self}

        for model in models:
            try:
                object = model.objects.get(**query)
            except model.DoesNotExist:
                pass
            else:
                return object

        if object == None:
            raise RelatedObjectException
        return object

This is a method used by BaseMedium.