Just to be sure though, I don't need to call save on the instances
unless I need to emit the save events right? Reading the manual:
Be aware that the update() method is converted directly to an SQL
statement. It is a bulk operation for direct updates. It doesn't run
any save() methods on your models, or emit the pre_save or post_save
signals (which are a consequence of calling save()). If you want to
save every item in a QuerySet and make sure that the save() method
is called on each instance, you don't need any special function to
handle that. Just loop over them and call save():
It implies that it's running the UPDATE SQL command directly so the
data is getting changed regardless of if I invoke save() myself.
Correct?
On Sun, Jul 11, 2010 at 2:03 AM, euan.godd...@googlemail.com
<euan.godd...@gmail.com> wrote:
> 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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