On Sat, 2008-02-23 at 15:22 -0800, Lee Hinde wrote:
> Hi;
>
> For an app that describes Wine Competitions, I have a model that
> includes:
>
> class Awardswon(models.Model):
> competition = models.ForeignKey(Competition,
> verbose_name="Competition")
> wine = models.ForeignKey(Wine, verbose_name="Wine")
> awardname = models.ForeignKey(Awardname, verbose_name='Award')
> score = models.CharField(blank=True,maxlength=10)
>
>
> class Wine(models.Model):
> class_number = models.CharField(blank=True, maxlength=100)
> wine_name = models.CharField(blank=True, maxlength=100,
> db_index=True)
> varietal = models.ManyToManyField(Varietal)
> <snip>
>
>
> class Varietal(models.Model):
> varietal = models.CharField(blank=False, maxlength=100)
>
>
> I'm able to display Wines that have won a specific award, with their
> associated varietal information:
>
>
> def loadWinners(request,award,comp):
> errorText = ""
> if comp and award:
> c = Competition.objects.get(pk=comp)
> aw =
> Awardswon.objects.select_related().filter(competition=comp).order_by('-
> score')
> aw = aw.filter(awardname=award)
> awn = Awardname.objects.get(pk=award)
> if len(aw)==0:
> errorText = "Nothing Found"
> else:
> aw=[]
> c=[]
> awn=[]
> return render_to_response("winelist.html",{"aw":aw,
> "mediaurl":settings.SITE_MEDIA_PREFIX,"error":errorText,"c":c,"awn":awn})
>
> And the view includes
>
> {% for wine_aw in aw %}
> <li><a href="/wine/{{wine.wine.id}}">{{wine_aw.wine}}</a>
> ({{wine_aw.wine.varietal.all|join:","}}) {{wine_aw.score}}</li>
> {% endfor %}
>
> Earlier I'd added StatsMiddleware and followng the author's discussion
> learned about select_related. (Thanks).
>
> But, using that tool, it seems that the many-to-many data isn't loaded
> in the initial query. That is, django does a query for each awarded
> wine to get the related varietals.
>
> Is there a way to load the varietals as part of the initial query?
Not without writing custom SQL at the moment. select_related() only
follows non-null many-to-one relations. One day we might add this
support (many-to-many, one-to-many, etc), but we need to build in "don't
shoot yourself in the foot" protection so that people don't accidentally
try to query multiple such relations at once, for example.
Malcolm
--
He who laughs last thinks slowest.
http://www.pointy-stick.com/blog/
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