On Wed, 2007-07-18 at 09:48 +0000, omat wrote:
> Hi all,
>
> Recently, it occurred to me that, defining some abstract models for
> some types of applications would make my code significantly DRYer, by
> eliminating duplication of common fields, model methods and manager
> methods.
>
> Examples of common groups of applications are:
>
> - Orthogonal applications, that is, applications that hold content
> related to other applications, such as tags.
> - Hierarchically categorized content
> - User contributed content
>
> There had been some discussion on "model inheritance" on the
> developers' list about a year ago, but this is basically for content
> that is hierarchically related, such as "carnivores are subclass of
> animals" and would be defined as:
>
> class Carnivore(Animal)
>
> But what I want is a bit different (and simpler). For example, for
> hierarchical content, define a Node() base model class as follows:
>
> class Node(models.Model):
> parent = models.ForeignKey('self')
> objects = NodeManager()
>
> And a manager class as follows:
>
> class NodeManager(models.Manager):
> def get_children(self, node):
> return self.filter(parent = node)
>
> def get_neighbours(self, node):
> return self.filter(parent = node.parent).exclude(id = node.id)
>
> Whenever a model holds hierarchical content, subclass Node(), like
> that:
>
> class MenuItem(Node):
> title = models.CharField(maxlength=50)
>
>
> I only tested with the Ortho() model class which can be found in the
> thread I started yesterday, before the issues appeared to be more
> complicated than I thought:
> http://groups.google.com/group/django-users/browse_thread/thread/975d56dff34a8784
>
> When 'syncdb'ed, it created a single table with the methods and fields
> from the base class included. With this approach, base class' fields
> and methods can be accessed with dot notation.
>
> But:
> - relations in the base class does not work, i.e. I cannot say "parent
> = models.ForeignKey('self')"
> - you cannot access the manager, but you can refer to the manager from
> within the subclass, so not a big issue
> - django goodies such as "get_object_or_404" fail with " Programming
> Error: relation "models_[basemodel]" does not exist", i.e. requires to
> have the table for the base class. But if you avoid using those
> goodies, and go with simple api methods, it works.
> - admin application frequently compşains with the same "Programming
> Error"
>
>
> Is there a way to achieve that kind of inheritance?
You can't do it at the moment, but it's one of the completion
requirements for model inheritance. It's exactly the abstract base class
scenario: all the parent model fields will be installed in the child as
though they were defined there. Parents will (possibly) never exist in
isolation. I say "possibly", because it's not clear at the moment if the
declaration that the parent class is abstract is a property of the
parent (a la C++) or the child.
So it's a "coming soon" feature.
Regards,
Malcolm
--
A clear conscience is usually the sign of a bad memory.
http://www.pointy-stick.com/blog/
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