Creating an unlimited forum hierarchy in Django.

Question

Hello,

I'm trying to design models for a forum I wish to create in Django.

So far I have:

class Forum(models.Model):
    name = models.CharField(max_length=255)
    slug = models.SlugField(max_length=150)
    description = models.TextField()

    def __unicode__(self):
        return self.name

class SubForum(models.Model):
    parent_forum = models.ForeignKey('Forum', related_name='forums')
    parent = models.ForeignKey('self', blank=True, null=True, related_name='children')
    name = models.CharField(max_length=300)
    slug = models.SlugField(max_length=150)
    description = models.TextField()

    def __unicode__(self):
        if self.parent:
            return u'%s: %s - %s' % (self.parent_forum.name,
                                     self.parent.name,
                                     self.name)
        return u'%s: %s' % (self.parent_forum.name, self.name)

This works for the most part as I am able to choose a parent category although I am not sure how to select parent of a child's parent. For example if I had the following:

Grandparent -> Parent -> Child

How would I select Grandparent from Child?

This kind of hierarchy also makes the Django admin rather messy as it doesn't cascade in an orderly fashion. Do I have to build the entire admin from scratch in order to organise this into a usable interface?

Lastly the __unicode__ function in the SubForum model allows me to print the parent but what about a grandparent. Can I get __unicode__ to print all ancestors?

Thanks.

Answer 1

I would simply create one Forum model that can either have another Forum as it's parent, or have a null parent. Then if you want to print out all parents you could use a while loop like this pseudocode:

heirarchy = ""
f = this forum model
while f.parent is not null:
   heirarchy.append(f.parent.name)
   f = f.parent

to answer the other part of your question, in order to select a child while knowing the parent id, you would query Django's object structure like so:

Forum.objects.filter(parent=PARENT_ID)

and to get the grandparent of a child you would do:

forum_object.parent.parent

Answer 2

What you want to do is to build a tree, or better yet hierarchy and traverse it. I suggest using MPTT over adjacency list model to build it. Both are techniques for easy manipulation and storage of such structures.

As for django implementation of MPTT try django-mptt.

Misc: nice article hierarchical data database design.