On Sat, 8 Oct 2011 16:30:37 -0500 Javier Guerra Giraldez <jav...@guerrag.com> wrote:
> On Sat, Oct 8, 2011 at 8:18 AM, Michal Petrucha <michal.petru...@ksp.sk> > wrote: > > Behind the scenes, Django does a JOIN. Its type (INNER od LEFT OUTER) > > is decided based on whether the left-hand column (your ForeignKey > > field) is allowed to be NULL or not. > > the weird part, is that is takes the same choice on reverse > relationships, which isn't necessarily appropriate. > > for example, model A has a foreign key to B. when reading A records > with select_related, it makes sense that if the FK is NULLable, it > uses a LEFT OUTER join while if it's not NULLable, use an INNER join. > when I have a B object, it's _really_ handy to have the reverse > relationship set up for me; but what if i want to select those B > objects that doesn't have any A? > > this works... > > B.objects.filter(a_set__isnull=True) > > but only if the FK from A to B has the null=True argument. > > and if I want to make the FK from A to B non-NULLable? maybe an A > record doesn't make any sense without B, but B without any A does. so > i don't put (null=True); so in the query above, i get an INNER join > followed by the condition that a.id IS NULL, and the result set is > empty. :-( > > the only solution i've found is to make the FK NULLable, even when it > doesn't make sense. I'd love to see a separate reverse_null=True > argument (by default reverse_null==null) on the FK, that would make > the reverse join a LEFT OUTER one while still prohibiting an A record > without B. > > am i missing something? This is part of the erroneous behavior fixed by a proposed patch to #16715 [1]. In this ticket, several other model relations lead to the wrong join type. Not having reverse relations be null-able is one of the observations. Maybe you, or some other developer, can review the proposed patch for this problem? The part of this mail, making reverse relations null-able is the more trivial part of the patch, and essentially a one-liner in django/db/models/sql/compiler.py. "new_nullable = f.null or None" in line 674 – as of r16928 – should be "new_nullable = True" since reverse relations should always be considered null-able. See the proposed patch (#16715) for details. Sebastian. [1] https://code.djangoproject.com/ticket/16715 -- You received this message because you are subscribed to the Google Groups "Django developers" group. To post to this group, send email to django-developers@googlegroups.com. To unsubscribe from this group, send email to django-developers+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-developers?hl=en.
