On Wed, 2007-10-31 at 11:09 -0400, George Vilches wrote: > Stated simply: > queryset.select_related().count() with no filter criteria generates a > wrong query across a ForeignKey relationship. > > The problem: A QuerySet operation that involves a .count() across a > ForeignKey relationship does not actually join in the ForeignKey tables > to do the select_related().count(), unless you force it to use criteria > on the ForeignKey table. If the ForeignKey match happens to be the > primary key in the foreign table as well (not in the example), then the > filter can't be on it, because QuerySet seems to optimize out the JOIN > (less important but possibly helpful info). > > > Example (create a new Django app and paste this right in, and then do > the rest from the shell): > > Assume I have two models in models.py: > > class Item(models.Model): > name = models.CharField(max_length=100) > group = models.IntegerField() > > class Assembly(models.Model): > desc = models.CharField(max_length=100) > item_group = models.ForeignKey(Item, to_field='group') > > > and they are populated with data like so: > > from qs.models import * > Item.objects.create(name='item1', group=1) > Item.objects.create(name='item2', group=1) > Item.objects.create(name='item3', group=1) > Item.objects.create(name='item4', group=2) > Item.objects.create(name='item5', group=2) > Item.objects.create(name='item6', group=2) > Assembly.objects.create(desc='as1', item_group_id=1) > Assembly.objects.create(desc='as1', item_group_id=2) > > > > Now, run these commands (I've included output from the Python interpreter): > > > >>> Assembly.objects.select_related() > [<Assembly: Assembly object>, <Assembly: Assembly object>, <Assembly: > Assembly object>, <Assembly: Assembly object>, <Assembly: Assembly > object>, <Assembly: Assembly object>] > >>> len(Assembly.objects.select_related()) > 6 > >>> Assembly.objects.select_related().count() > 2L > > > Since I'm using select_related(), I would expect it to follow the INNER > JOINs to do the .count(). I can force it to use the JOINs by doing this:
This is where you've made a mistake (aside from the to_field problem Karen pointed out). Your assumption is wrong. select_related() is only an optimisation as far as loading the data form the database. It should *never* change the result of a quantitative query like this. If it does, it would be a bug. Regards, Malcolm -- I intend to live forever - so far so good. http://www.pointy-stick.com/blog/ --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Django users" group. To post to this group, send email to django-users@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/django-users?hl=en -~----------~----~----~----~------~----~------~--~---
