On Mon, 2009-01-05 at 04:42 -0800, coan wrote:
> I have a model that looks something like this:
>
> class Collection(models.Model):
> user = models.ForeignKey(User)
> item = models.ForeignKey(Items)
> date = models.DateTimeField()
>
> I'm trying to retrieve a list of unique users ordered by the last time
> they added something to their collection.
> I would expect this to work:
> distinct_users = User.objects.order_by('collection__date').distinct()[:
> 5]
> This query returns a list of users, but the users are not distinct.
There can be more than one date associated with a particular user, so
whilst the user + date combinations are all unique, the users on their
own may not be.
> How do I do this the right way?
Right now, with trunk or Django 1.0, it's not particularly easy without
using custom SQL or extra(). Without looking too hard at the details, I
suspect it's probably possible with the aggregates code that will be in
Django 1.1, though (it's currently on track to be merged into trunk in
about a week).
One practical solution is to use the result set you're getting now and
filter it further in Python. Or do a filter on the User table and use
extra(select=...., order_by=...) to also select the maximum creation
date for each user and order by that.
Regards,
Malcolm
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