How can I handle dynamic calculated attributes in a model in Django?

Question

In Django I calculate the breadcrumb (a list of fathers) for an geographical object. Since it is not going to change very often, I am thinking of pre calculating it once the object is saved or initialized.

1.) What would be better? Which solution would have a better performance? To calculate it at _init_ or to calculate it when the object is saved (the object takes about 500-2000 characters in the DB)?

2.) I tried to overwrite the _init_ or save() methods but I don't know how to use attributes of the just saved object. Accessing *args, **kwargs did not work. How can I access them? Do I have to save, access the father and then save again?

3.) If I decide to save the breadcrumb. Whats the best way to do it? I used http://www.djangosnippets.org/snippets/1694/ and have crumb = PickledObjectField().

The model:

class GeoObject(models.Model):
    name = models.CharField('Name',max_length=30)
    father = models.ForeignKey('self', related_name = 'geo_objects')
    crumb = PickledObjectField()
    # more attributes...

Thats the method to calculate the attribute crumb()

def _breadcrumb(self):
    breadcrumb = [ ]
    x = self
    while True:
        x = x.father
        try:
            if hasattr(x, 'country'):
                breadcrumb.append(x.country)
            elif hasattr(x, 'region'):
                breadcrumb.append(x.region)
            elif hasattr(x, 'city'):
                breadcrumb.append(x.city)
            else:
                break
        except:
            break
    breadcrumb.reverse()
    return breadcrumb

Thats my save-Method:

def save(self,*args, **kwargs):
    # how can I access the father ob the object?
    father = self.father # does obviously not work
    father = kwargs['father'] # does not work either 

    # the breadcrumb gets calculated here
    self.crumb = self._breadcrumb(father)
    super(GeoObject, self).save(*args,**kwargs)

Please help me out. I am working on this for days now. Thank you.

Answer 1

By calling both the _breadcrumb method with x.father and assigning x = x.father in the beginning of the while loop you jump over one father. Try exchanging

self.crumb = self._breadcrumb(father) 

with

self.crumb = self._breadcrumb(self)

By defining _breadcrumb within the model class you can clean it up like this:

class GeoObject(models.Model):
    name = models.CharField('Name',max_length=30)
    father = models.ForeignKey('self', related_name = 'geo_objects')
    crumb = PickledObjectField()
    # more attributes...

    def _breadcrumb(self):
        ...
        return breadcrumb

    def save(self,*args, **kwargs):
        self.crumb = self._breadcrumb()
        super(GeoObject, self).save(*args,**kwargs)

For more complex hierachies I recomend django-treebeard