For these derived values, I usually have an attribute of the model that
is updated at save time. You could have an 'order_total' on your Order
model, and whenever you save an OrderLine, it updates the order_total on
its associated Order and saves the order. Then it just becomes another
attribute of the order (albeit denormalised - although that's not such a
bad thing in these cases as you'll save having to recalculate it every
time.) Check out slide 17 onwards on the Django Masterclass presentation
at http://toys.jacobian.org/presentations/2007/oscon/tutorial/
Hope that helps,
Mike
[EMAIL PROTECTED] wrote:
> Hi
>
> I have a fairly simple use-case which I feel should be achievable, but
> which I'm not able to pull off. What I want is the ability to modify
> the base query used by the admin to add some derived properties. For
> example, if you have Orders with corresponding OrderLines, it's useful
> to add the subtotal for each order in the order list. To achieve this
> I tried the following:
>
> class OrderAdminManager(models.Manager):
> def get_query_set(self):
> return super(OrderAdminManager, self).get_query_set().extra(
> select={'order_total': '(select sum(quantity * item_price)
> from webshop_orderline where order_id = webshop_order.id)'})
>
> And in the inner Admin class I added this line:
> manager = OrderAdminManager()
>
> The net result of this is that the query result contains the
> order_total column, and I can display it in the admin, but only if I
> do so using a custom method, which means no sorting and no filtering.
> If I try to add "order_total" as a non-editable field it blows up
> when Django queries for webshop.order_total.
>
> Any ideas?
>
>
> >
>
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