class SelfForeignKey(models.ForeignKey):
    def pre_save(self, instance, add):
        manager = instance.__class__.objects
        ancestor_id = getattr(instance, self.attname)
        while ancestor_id is not None:
            if ancestor_id == instance.id:
                return None
            ancestor = manager.get(id=ancestor_id)
            ancestor_id = getattr(ancestor, self.attname)
        return getattr(instance, self.attname)

# used like this:

class Category(models.Model):
    name = models.CharField(max_length=20)
    parent = SelfForeignKey('self', related_name='child_category_set', ...)

Data modelling Back to the Future

grandfather > father > son > father > ...

Using this field instead of the standard ForeignKey will ensure that no model instance is saved that has itself as an ancestor, breaking the relationship if it does.

(Enhancements: I am sure one would want to better enhance this with appropriate error handling instead of silently disconnecting the relationship. And the relevant forms ought not show ancestors in the field's widget to reduce the chances of this happening in the first place.)

Author:: jamesgpearce
Posted:: February 1, 2010
Language:: Python
Django Version:: 1.1
Tags:: loop pre_save foreign recursion
Score:: 0 (after 0 ratings)

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SelfForeignKey to prevent hierarchical loops

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Data modelling Back to the Future

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