Django-mptt and multiple parents?

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,