On Jul 27, 1:53 pm, Josh <jos.carpente...@yahoo.com> wrote:
Daniel already answered, just a couple observations:
> 1. the id of the record is not known.
Given your question ("update (...) fields in a model", which I assume
mean : "update (...) fields in a model _instance_"), you obviously
know the pk either directly (passed as argument etc), indirectly (you
have some lookup that will retrieve the model instance), or of course
thru the instance itself (if you already have it).
> 2. Using the samples in the query-documentation. Create another query to get
> a single object in a queryset using:
> n = SomeModel.objects.get(id=myid)
> Somemodel.objects.filter(pk=n).update(somefield=newfielddata)
the "pk" lookup shortcut expected a litteral value, not a model
instance, so, assuming your model's id field is the pk (which is
usually the case), this should be:
n = SomeModel.objects.get(id=myid)
Somemodel.objects.filter(pk=n.id).update(somefield=newfielddata)
which is a overcomplicated (and rather inefficient) way to write:
Somemodel.objects.filter(pk=myid).update(somefield=newfielddata)
> m.save()
Did you mean 'n' ? If yes, a call to n.save() at this point will
overwrite the previous update.
> I was thinking to try a different approach. When a record exists, retrieve
> all existing data and add this to the .save(). This seems like a pretty
> cumbersome way to achieve updating a few fields.
Indeed !-)
> What is the best way to
> achieve this?
Depends on quite a few things. Queryset.update is atomic - which can
avoids some problems with concurrent access - and is quite fast. OTHO,
it won't trigger the pre_save and post_save signals, nor even call on
your model's save method, so if your code relies on either a custom
save method and/or pre/post save signals, you might screw it up.
Most often, you'll be safer using Daniel's solution for most things
and only using Queryset.update for batch updates if and when you know
you don't have any python code triggered by yourmodel.save()
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