On Sat, May 16, 2009 at 6:14 PM, Alex Rades <alera...@gmail.com> wrote:
>
> Hi,
> I have a model which looks like:
>
> class Person(models.Model):
> friends = models.ManyToManyField("self", through="Friendship")
>
> class Friendship(models.Model):
> person_a = models.ForeignKey(Person)
> person_b = models.ForeignKey(Person, related_name="_unused_")
>
> Now, as mentioned in:
>
> http://docs.djangoproject.com/en/dev/topics/db/models/#intermediary-manytomany
>
> For this to work, you also /have/ to use symmetrical=False in the
> ManyToManyField, but frankly I don't understand why.
> Do you have an explanation?
When you have a symmetrical m2m relation to self, Django needs to
store 2 rows in the m2m table to fully represent the relation - that
is, when you store a 'friendship' between Alice and Bob, Django stores
the relationship (alice, bob) and the relationship (bob, alice).
This poses a problem when you are using an intermediate m2m table:
what happens to the other data in the m2m table? Do you duplicate the
intermediate data? How do you guarantee consistency? If I modify the
m2m relation in one direction, what updates the row in the other
direction?
If anyone comes up with a creative way to address this issue, I'm
certainly willing to entertain those ideas. However, in the interim,
Django takes the conservative path, preventing behaviour that is
predictably problematic.
Yours,
Russ Magee %-)
--~--~---------~--~----~------------~-------~--~----~
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
django-users+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/django-users?hl=en
-~----------~----~----~----~------~----~------~--~---
