Django doesn't support the query type you're trying to do. You've
already discovered F which is about as complicated as it gets, bt the
update syntax is pretty much limited to setting a field's value to a
consant.
Looping over the minimum set of things and calling save on each one if
your only option without resorting to custom SQL.
Euan
On 11 July, 07:44, Doug Warren <doug.war...@gmail.com> wrote:
> Can Django support an update by setting a field to the least of two args?
>
> I have a model that looks like:
>
> class Bar
> maxval = models.FloatField()
> rate = models.FloatField()
>
> class Foo(models.Model):
> bar = models.ForeignKey(Bar)
> current = models.FloatField()
>
> and i'd like to increase current up to max by rate for every case
> where it's not yet the max...
>
> IE: I'd like to do something like:
> Foo.objects.filter(current__lt=F('bar__maxval')).update(current=least(Q('current')+Q('bar__rate'),Q('bar__maxval')))
>
> But that fails because you can't add 2 query sets together, can't join
> field referenecs in an update, and I don't see support for 'least'
> What I've done instead is rewritten it to iterate over bars each time
> filtering on foo like:
>
> set = foo.objects.filter(bar=bar).filter(current__lt=bar.max)
> and then compute the min amount of current to hit max this update, and
> update that, then iterate over what remains and update each one
> individually, ie:
>
> canreachfull = bar.max - bar.rate
> set.filter(current__gte=canreachfull).update(current=bar.rate)
> set = foo.objects.filter(bar=bar).filter(current__lt=bar.max)
> for record in set:
> newcurrent = record.current + bar.max
> record.update(current=newcurrent)
>
> Is there a better way to do this?
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