Unfortunately the Django ORM's doesn't support conditionnal
aggregates<https://code.djangoproject.com/ticket/11305>
.
Le mardi 24 septembre 2013 16:50:51 UTC-4, Victor Hooi a écrit :
>
> Hi,
>
> I'm trying to use aggregation to count the number of "likes" on an item.
>
> The likes for an item are stored in a related model, called LikeStatus:
>
> class Item(models.Model):
>> ...
>> class ItemLikeStatus(models.Model):
>> LIKE_STATUS_CHOICES = (
>> ('L', 'Liked'),
>> ('U', 'Unliked'),
>> ('A', 'Abstained (no vote)'),
>> ('N', 'Not seen yet'),
>> )
>> user = models.ForeignKey(User)
>> item = models.ForeignKey(Item)
>> liked_status = models.CharField(max_length=1,
>> choices=LIKE_STATUS_CHOICES)
>
>
> I'm using the following aggregation:
>
> items_by_popularity =
>> Item.objects.all().annotate(Count('itemlikestatus')).order_by('-itemlikestatus__count')[:number_requested]
>
>
> However, I would like to count up only the like statuses where the
> liked_status code is "L".
>
> My thoughts are that I can iterate through the entire Item set myself and
> do a filter on the related itemlikestatus for each Item, and produce the
> aggregation by hand. However, that seems like a silly way to do it, and
> doesn't really use the database.
>
> I was wondering if there's a smarter way to still use aggregation, but
> filter out to only count a subset of related ItemLikeStatus?
>
> Cheers,
> Victor
>
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