How do you define a specific ordering in Django
Specifically, if I have a
QuerySet like so:
['a10', 'a1', 'a2'].
Regular order (using
Whatever.objects.order_by('someField')) will give me
['a1', 'a10', 'a2'], while I am looking for:
['a1', 'a2', 'a10'].
What is the proper way to define my own ordering technique?
As far as I'm aware, there's no way to specify database-side ordering in this way as it would be too backend-specific. You may wish to resort to good old-fashioned Python sorting:
class Foo(models.Model): name = models.CharField(max_length=128) Foo.objects.create(name='a10') Foo.objects.create(name='a1') Foo.objects.create(name='a2') ordered = sorted(Foo.objects.all(), key=lambda n: (n, int(n[1:]))) print ordered # yields a1, a2, 10
If you find yourself needing this kind of sorting a lot, I'd recommend making a custom
models.Manager subclass for your model that performs the ordering. Something like:
class FooManager(models.Manager): def in_a_number_order(self, *args, **kwargs): qs = self.get_query_set().filter(*args, **kwargs) return sorted(qs, key=lambda n: (n, int(n[1:]))) class Foo(models.Model): ... as before ... objects = FooManager() print Foo.objects.in_a_number_order() print Foo.objects.in_a_number_order(id__in=[5, 4, 3]) # or any filtering expression
@Jarret's answer (do the sort in Python) works great for simple cases. As soon as you have a large table and want to, say, pull only the first page of results sorted in a certain way, this approach breaks (you have to pull every single row from the database before you can do the sort). At that point I would look into adding a denormalized "sort" field which you populate from the "name" field at save-time, that you can sort on at the DB-level in the usual way.