On Tue, Aug 18, 2009 at 10:48 PM, mettwoch<mettw...@pt.lu> wrote:
>
> The whole thing behind that it is: I'm developing an invoicing/
> inventory application and I run into performance problems that might
> require using raw SQL and admittedly I'm not an SQL expert and I
> dislike the idea of doing so.
>
> I thought more on an approach of "SQL view" than denormalisation or a
> pseudoattribute. I've also read a little about F() and the extra()
> function to inject SQL, but that's to be done for every query while
> I'm more interested on having the calculation (business rule) defined
> once in the model.
>
> I could also access a view in PostGres where 'total' is defined as
> 'price*quantity' but as far as I know (and my SQL knowledge is rather
> poor), views are not updatable.
>
> The example was a quick and dirty draft and should read:
>
> from django.db import models
>
> class InvoiceLine(models.Model):
> price = models.DecimalField()
> quantity = models.DecimalField()
> total = models.ExpressionField('price * quantity')
>
> The SQL looks something like this for InvoiceLine.objects.all():
>
> SELECT price, quantity, price * quantity AS total FROM invoiceline;
Ok - that's a little clearer.
This proposal has overlap with two others:
Firstly, #10972 - Using expressions with annotate(). This would allow
you to issue the following query:
InvoiceLine.objects.aggregate(Sum(F('price') * F('quantity'))
Secondly, an idea that doesn't have a ticket (which is something I
should probably correct) - annotating non-aggregate clauses. That is,
you should be able to do the following:
InvoiceLine.objects.annotate(total=F('price') *
F('quantity')).aggregate(Sum('total'))
This creates a 'total' column and attribute that isn't part of an
aggregate clause
With an implementation of this second idea, you don't need to have an
ExpressionField - you can just override the manager to have a default
query set that adds the annotation.
Both of these should be possible without too much effort - it's just a
matter of someone writing the code.
Yours,
Russ Magee %-)
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