Django-mptt and some parents?

Question

For several weeks now, I have been banging my head on the table to understand that the time has come to ask for help.

I am trying to implement a database structure that has hierarchical parts data for assemblies. My main problem is trying to assign one "subassembly" to another "assembly" / tree. Turning to the examples of trees below, I have no problem creating and working with assemblies 1 and 2. But when I do assembly 3, I get several objects that return errors when I call nodes (which I understand based on how I'm trying).

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 tried:

 #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 create very simple trees and bind to more detailed data through Part FK. I need to be able to query any assembly and look at the descendants of it, so although I can call assembly 3, I cannot name any of the children that were on several trees.

For what I do, from any point in the tree, the descent will always be the same, i.e. 1.2 will always have a child 1.2.1, but growth can change, that is, 1.2 can have parents 1 and / or 3.

The problem is that Assembly.parent is a ForeignKey as it limits it to a single 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 like a difficult subject to try to explain clearly! thanks

---- EDIT ----

I realized that I needed a directed a-cyclic graph (DAG) model.

I don’t have a specific solution yet, but when I find out or ask a DAG question, I will try to post the link here.

---- EDIT 2 ----

django-treebeard-dag

django-dag

I found these two very small projects. At first I worked with the version of "treebeard", and then switched to django-dag for my project. Feel free to PM with basic usage questions and I will 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 the tree in data structures is that the nodes have one parent, or the node is the root of the tree.

Trees are a generalized form of Graphics . I do not know any Django application that will help you deal with them.

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