ManyToMany relationship can't be saved

[Adam Smith]

As I was learning the Django Docs (https://docs.djangoproject.com/en/1.6/topics/db/models/#extra-fields-on-many-to-many-relationships), I found the following code not working. Is it a bug? At least the result is unexpected.

ringo = Person(name="Ringo")

beatles = Group(name="Beatles")

m1 = Membership(person=ringo, group=beatles,

                date_joined=date.today(),

                reason='whatever')

ringo.save(), beatles.save()

print m1.person.id, m1.group.id  # yes, they have values.

m1.save()  # IntegrityError: membership.person_id may not be NULL

Why can't m1 be save, since both its foreign key fields have been save successfully?

Thanks.

 

[Curtis Maloney]

At first glance, I'd guess it's because the PK for ringo and beatles haven't been updated in the instances you have.

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[Shai Berger]

m1 actually has more "fields" than meet the eye: besides person and group,
there are also person_id and group_id (and these are the attributes that go
into the database). The *_id attributes are set when the object attributes are
set -- that is, the assignment

m1.person = ringo # implicit in your code, executed by the initializer

causes also

m1.person_id = ringo.id

In your sample, this was done before ringo and beatles were saved, so the id
value was still None.

A case can be made for updating the ids for FK fields before saving an object,
which will make your problem go away. Off hand, I'm -0 on that -- it reeks of
"action at a distance", on pure instinct I'd say it will probably cause more
problems than it solves.

Shai.