On Sun, Oct 12, 2008 at 2:12 PM, Peter Krantz <[EMAIL PROTECTED]>wrote:
>
> Hi!
>
> I have a model like this:
>
> class Thing(models.Model):
> x = models.PositiveIntegerField()
> y = models.PositiveIntegerField()
>
> def z(self):
> return self.x / self.y * A_CONSTANT
>
>
> In a view I would like to retrieve a collection of Things ordered by z
> (the computed field). I expected the order_by method to do the trick
> like this:
>
> things_collection = Thing.objects.order_by('z')
>
> ...but order_by will only work for fields actually stored in the
> database (the above gives a "Cannot resolve keyword 'z' into field").
> What would be the best way to implement a custom order_by that allows
> ordering of computed model fields? Or have I missed some feature for
> this in Django?
>
>
order_by on a QuerySet translates to 'ORDER BY' in the SQL SELECT sent to
the database. So you have to order_by something known to the database. You
can force the calculation & result to be part o fthe SQL select and thus be
something you can specify in order_by by using extra():
Thing.objects.extra(select={"calcz" : "x / y * %d" %
A_CONSTANT},order_by=["calcz"])
Whether that's "best" depends on how complicated your actual computation is,
how many rows you have in your table, how often you do this select vs.
update rows, etc. Alternatively you could store z directly in the database
and avoid having to do the computation for every row when you do the
select. But then you have to ensure z stays in sync with changes to x & y.
Karen
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