Ah, thanks for the values() hint. It seems to work decently. I don't
really need to access related model fields, I just want a list of
unique group objects associated with a given musician's activities.
The above becomes instead:
qset = theMusician.activity_set.all().values('group').distinct()
# returns list of {'group': id} without duplicates
grouplist = [Group.objects.get(id=valuesdict['group']) for valuesdict
in qset]
Or maybe it would be better to leave the duplicates testing in python:
grouplist = [act.group for act in theMusician.activity_set.all()]
group_ids = []
for group in grouplist[:]:
if group.id in group_ids:
grouplist.remove(group)
else:
group_ids.append(group.id)
Why doesn't set(grouplist) remove the duplicates, anyway?
Do these both make sense? Would one hit the database less than the
other?
Thanks again,
Andy
On Oct 31, 11:01 pm, Malcolm Tredinnick <[EMAIL PROTECTED]>
wrote:
> On Wed, 2007-10-31 at 16:46 -0700, Andy Brody wrote:
> > Hi all,
>
> > I'm using an intermediate table of Activities to relate Musicians,
> > Instruments, and Groups. Is there a good way to select groups from the
> > musician's activities? I've been doing stuff like [act.group for act
> > in theMusician.activity_set.all()]. This works pretty well, except
> > that there are duplicates if a musician plays more than one instrument
> > in a group, and merely doing set( the_above ) doesn't get rid of them
> > even though group1==group2 evaluates to True.
>
> > Is there some better way to do this? It seems strange to be trying to
> > remove the duplicates at this level rather than at the SQL level with
> > a distinct(). The resulting SQL (which I don't want to write myself)
>
> Well, there's the first problem. If Django's ORM can't do something --
> we aren't trying to completely replicate SQL, after all, so there will
> always be some cases that can't be done in Python -- then writing custom
> SQL is not just possible, it's recommended.
>
> > should be something like:
>
> > SELECT DISTINCT *
> > FROM `app_group`
> > WHERE `id`
> > IN (
>
> > SELECT `group_id`
> > FROM `app_activity`
> > WHERE `musician_id` = 1
> > )
>
> Ideally, you want to be able to write something
> involving .values(...).distinct(). Unfortunately, referencing related
> model fields via values() isn't possible yet. It will be possible when
> the queryset-refactor work is completed (this is ticket #5768).
>
> Regards,
> Malcolm
>
> --
> The early bird may get the worm, but the second mouse gets the
> cheese.http://www.pointy-stick.com/blog/
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