Question

I’ve been banging my head against the desk for a couple weeks on this problem, so I figure it may be time to seek some help.

I’m trying to implement a database structure which has hierarchical data of parts for assemblies. My main problem lies with trying to assign one “subassembly” to another “assembly”/tree. Refering to the example trees below- I have no problem creating and working with assembly 1 and 2. But when I make assembly 3, I get multiple objects returned errors when I call up the subassemblies (which I understand base on the way I’m attempting).

assembly 1:    assembly 2:     assembly 3:
1.1            2.1             2.1
- 1.1.1        - 2.1.1         - 2.1.1
1.2            2.2             1.2
- 1.2.1        - 2.2.1         - 1.2.1               

Here is the model I’ve been trying:

#models.py snippet
class Part(models.Model):
        part_name = models.CharField(max_length=30, primary_key=True)
        description = models.TextField(max_length=500, blank=True, null=True)
        revision = models.CharField(max_length=10, blank=True, null=True)

        def __unicode__(self):
                return u'%s' % (self.part_name)

class Assembly(MPTTModel):
        name = models.ForeignKey(Part)
        parent = models.ForeignKey('self', null=True, blank=True, related_name='children')

        def __unicode__(self):
                return u'%s' % (self.name)

#views.py snippet
def assembly_details(request, assembly_name):
        context_instance=RequestContext(request)
        assembly = Assembly.objects.get(name=assembly_name)
        descendants_list = assembly.get_descendants(include_self=False)
        return render_to_response('assembly_details.html', locals(), context_instance,)

So basically I’m creating very basic trees and linking to the more detailed data through the Part FK. I need to be able to query any assembly and look at it’s descendants- so although I can call assembly 3, I can’t call any of the children that have been in multiple trees.

For what I’m doing, from any given point in the tree, going down will always be the same, i.e. 1.2 will always have a child of 1.2.1, but going up can change, i.e 1.2 can have parents of 1 and/or 3.

The problem is having the Assembly.parent be a ForeignKey as it limits it to one value. Any ideas on some solutions or things to try?

Please let me know if you want to see additional code or ask questions. This seems to be a difficult subject to try to clearly explain! Thanks

—-EDIT—-

I’ve figured out that I need a directed a-cyclic graph (DAG) model.

I don’t have a specific solution yet, but when I figure it out or ask a DAG question, I’ll try to post a link here.

—-EDIT 2—-

django-treebeard-dag

django-dag

I found these two very small projects. I was working with the “treebeard” version originally, then switched over to django-dag for my project. Feel free to PM with questions about basic usage, and I’ll see if I can help.

Answer 1

I think Django-mptt is the wrong tool for this particular job. It deals with trees, and part of being a tree in Data Structures is that nodes have one parent, or the node is the root of the tree.

Trees are a generalized form of Graphs. I know of no Django app that’ll help you handle those.

You may have to resort to maintaining your own ManyToMany relationships and forgo the convenience of Django-mptt.