Django model inheritance using mptt

Question

I'm bulding a model with nested tree classification. A vocabulary Entity is inherited from an abstract base class TreeVocabulary. There is also a class SpecialEntity, which inherits from Entity. In SpecialEntity there should be few additional fields.

Entity and SpecialEntity should both be trees, for which I use mptt http://django-mptt.github.com/django-mptt/. In Entity there should be a record, which has children in the SpecialEntity (those children are root elements in the SpecialEntity).

This is how i imagine this:

class Vocabulary(models.Model):
    name= models.CharField(max_length=300)
    order= models.IntegerField(default=100)
    class Meta:
        abstract= True

class SpecialType(Vocabulary):

class TreeVocabulary(MPTTModel):
    parent= TreeForeignKey('self', null=True, blank=True,
        related_name='children', limit_choices_to=Q(parent__isnull=True))
    class MPTTMeta:
        order_insertion_by= ('name',)
    class Meta:
        abstract= True

class Entity(TreeVocabulary):

class SpecialEntity(Entity):
    code= models.CharField(max_length=50)
    type= models.ForeignKey(SpecialType)
    class Meta:
        ordering= ('code',)

Now, the problem is that for some reason SpecialEntity escapes mptt: sqlall shows a plain table without parent_id in it. Although it's present in Entity, which directly inherits from TreeVocabulary.

Is it a bug in django-mptt? Or maybe it is a bad design? I'm not asking to design it for me, but rather to point in the right direction

Thanks in advance!

Answer 1

Ok, answering my own question after a short investigation.

Multi-table inheritance is possible in mptt, but all mptt fields (parent, lft, rght, level, etc.) for any child should be stored in the one (obviously, parent) table.

Taking into consideration the principles of Modified Preorder Tree Traversal, it is reasonable.

For the example in my question in the db there will be created:

• specialtype (plain table)

• entity - tree structure with the mptt fields, containing data for both models Entity and SpecialEntity

• specialentity - plain table with foreign key to entity, containing only fields specific to SpecialEntity