The wrong rows being deleted is a possibility. Here's an example:
class Through(models.Model):
two = models.ForeignKey('Model2', to_field='spot')
one = models.ForeignKey('Model1')
class Model1(models.Model):
name = models.CharField(max_length=100)
class Model2(models.Model):
spot = models.IntegerField(unique=True)
ones = models.ManyToManyField(Model1, through=Through)
# Create objects and relations
m = Model1(name='test')
m2 = Model2(spot=2, pk=1)
m3 = Model2(spot=1, pk=2)
m.save()
m2.save()
m3.save()
Through(two=m2, one=m).save()
Through(two=m3, one=m).save()
# Both have ones
m2.ones.all()
m3.ones.all()
m2.ones.clear()
# Still has ones, didn't get deleted when it should have
m2.ones.all()
# This got deleted instead
m3.ones.all()
It's not exactly a real world example though i think it demonstrates that
data loss is a possibility.
Monday, August 27, 2012 5:33:03 PM UTC-7, Russell Keith-Magee wrote:
> On Tue, Aug 28, 2012 at 3:11 AM, peter <[email protected] <javascript:>>
> wrote:
> > I opened this ticket (https://code.djangoproject.com/ticket/18823) on
> the
> > Trac but thought i'd bring it up here to increase the likelihood of it
> > getting noticed. In short things don't quite work right when you have a
> m2m
> > field that uses a through model that uses a to_field for it's foreign
> key.
> > If someone wouldn't mind reviewing the patch and giving me some
> feedback,
> > I'd like to help get this issue fixed.
> >
> > Assuming a fix in some form is accepted, I was also wondering if this
> bug
> > fix would be considered important enough to backport to bugfix branches?
> Or
> > will we need to wait for the next major release to get the fix in an
> > official release.
>
> Generally speaking, we don't backport bug fixes unless they relate to:
>
> * Security issues
>
> * Major problems with newly added features
>
> * Problems with the potential to cause catastrophic data los -- i.e.,
> accidentally deleting/dropping data, or deleting/dropping the wrong
> data.
>
> A rule of thumb: If we'd discovered this feature the day before we cut
> a major release, would we have held back the release?
>
> In this case, it's not a security issue, and the code has had this
> problem for several years; we certainly aim for being bug free, but
> we've evidently survived with this bug for some time without any
> serious problems. However, you *might* be able to make the case that
> catastrophic data loss is possible. From an initial analysis, it seems
> like it might be possible to accidentally delete the wrong through
> model. If you can provide a clear example of a case where this would
> happen, then yes, a backport is likely.
>
> Yours,
> Russ Magee %-)
>
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