thanks for the thorough reply! I now understand how to proceed.
However, I'm not convinced that distinct shouldn't do this for me...
but I understand this is a philosophical point, and I put my trust in
the django developers to get it right (just like I do with the
american government... )
On Dec 4, 9:24 pm, Malcolm Tredinnick <[EMAIL PROTECTED]>
wrote:
> On Thu, 2008-12-04 at 19:09 -0800, ryan wrote:
> > For instance, I have two objects:
> > 1) Blog
> > 2) Entry with a ForeignKey to Blog, and a Date field titled DateAdded
>
> > Now, I want to list all my blogs, ordering by the most recent added
> > entry.
>
> > I tried:
> > blog.objects.all().sort_by('entry__dateAdded').distinct()
>
> > But the result was that is a given blog had more than 1 entry, then it
> > shows up in the resulting queryset more than once. It appears distinct
> > () is doing nothing in this case.
>
> The distinct() call is working correctly (you're right that it is
> effectively doing nothing, wrong to say it "doesn't work"). The problem
> is that you are ordering by something that has multiple values and each
> value contributes one output row. There is nothing in your ordering
> statement to say that the database should only use the *maximum* value
> of dateAdded for each entry. The only reasonable way to interpret
> ordering by multi-valued fields is to treat each one as generating a
> different output row, since otherwise we might as well just pick one at
> random (the SQL specification agrees with that interpretation by the way
> -- which is why Django does it. Only MySQL allows effective ordering by
> columns not in the output list in this case and it's very non-standard).
>
> Refer to the massive callout box in the documentation for distinct() for
> a more SQL-specific
> explanation:http://docs.djangoproject.com/en/dev/ref/models/querysets/#distinct
>
> You could achieve the effect you're after by using extra(select=...,
> order_by=...) to select the maximum date added value as an extra
> column.
>
> Alternatively (and this would require some fairly deep poking into Query
> internals, so probably not recommended) you could add some extra stuff
> to the where-clause to say "where dateAdded = (Select max(dateAdded)
> from ...)". Possible some extra(where="....") usage could get you most
> of the way there on that one.
>
> Regards,
> Malcolm
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