django - QuerySet recursive order by method

Question

I may have a classic problem, but I didn't find any snippet allowing me to do it.

I want to sort this model by its fullname.

class ProductType(models.Model):
    parent   = models.ForeignKey('self', related_name='child_set')
    name     = models.CharField(max_length=128)

    def get_fullname(self):
        if self.parent is None:
            return self.name
        return u'%s - %s' % (unicode(self.parent), self.name)
    fullname = property(get_fullname)

1. I tried sorting by "parent", got infinite loop error. "parent__id" did not sort well.

2. I could not understand how to use annotate() for concatenating string fields.

3. I added a custom manager with sorted(), but it returns a list object and prevents my forms.ModelChoiceField to work.

Here's the sort

def all(self):
    return sorted(super(ProductTypeManager, self), key=lambda o: o.fullname)

What else is there in the djangonic jungle ? Thanks for your help.

Answer 1

This might work:

ProductType.objects.alL().order_by('parent__name', 'name')

Answer 2

I would probably create a denomalised field and order on that. Depending on your preferences you might wnat to override .save(), or use a signal to poplate the denormalised field.

class ProductType(models.Model):
    parent   = models.ForeignKey('self', related_name='child_set')
    name     = models.CharField(max_length=128)
    full_name     = models.CharField(max_length=128*4)

    def save(self, *args, **kwargs):
        if not full_name:
            self.full_name = self.get_fullname()
        super(ProductType, self).save(*args, **kwargs)


    def get_fullname(self):
        if self.parent is None:
            return self.name
        return u'%s - %s' % (unicode(self.parent), self.name)

Then do a normal order by full_name

Answer 3

agreed :

ProductType.objects.alL().order_by('parent__name', 'name')

http://docs.djangoproject.com/en/dev/ref/models/querysets/#order-by-fields

Answer 4

I would definitely explore the route you mentioned as 1) above:

ProductType.objects.order_by('parent__name', 'name')

Why is it erroring with an infinite loop? Is your example data referencing itself?

Answer 5

Or, if what you're trying to do is to generate a tree structure, have a look at django-mptt. It also allows for ordering on a manually set order.