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?
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