On Fri, Jul 4, 2014 at 4:42 PM, Jon Dufresne <jon.dufre...@gmail.com> wrote:
> Suppose I have the following models:
>
> ---
> class Container(models.Model):
> pass
>
> class Item(models.Model):
> container = models.ForeignKey(Container)
> previous = models.ForeignKey('self', null=True)
> current = models.BooleanField()
> flag = models.BooleanField()
> ---
>
[snip]
> A slow query is better than no query at all. Any particular reason
> this was removed? How would one model and query what I'm trying to
> achieve moving forward to avoid inefficient queries and exceptions?
>
Two ways come to mind:
* If it's only "flag" on the first item in a sequence then move "flag" to
be on present on Container also. A "first_flag" field. Could manage in a
signal or override Item.save(). That would let you not nest the query, and
could be more efficient.
* Build a list of relevant containers first:
Container.objects.filter(item__previous__isnull=True,
item__flag=True).values_list('pk', flat=True). Then
Items.objects.filter(current=True, container__in=flagged_containers). Not a
problem as long as your query doesn't grow too large for your DB (that is,
too many containers that might be considered)
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