On Tue, 2006-06-27 at 23:53 -0400, bsdlogical wrote:
> I've recently stumbled over a problem whose solution I can't find. I
> have a class with two fields that use the built-in User class (from
> django.contrib.auth.models) in a many-to-many relationship. However,
> when I try to access or modify those fields, django throws an error
> message. My class declaration is:
>
> class Organization(models.Model):
> name = models.CharField(maxlength=40)
> sponsor = models.ManyToManyField(User,
> related_name='sponsored_orgs', null=True)
> members = models.ManyToManyField(User,
> related_name='orgs_belonged_to', null=True)
> test = models.ForeignKey(User)
>
> The following code:
> o = Organization()
> o.sponsor
>
> causes an error after the second line:
> Traceback (most recent call last):
> File "<console>", line 1, in ?
> File
> "/usr/local/lib/python2.4/site-packages/django/db/models/fields/related.py",
> line 405, in __get__
> target_col_name=qn(self.field.m2m_reverse_name())
> File
> "/usr/local/lib/python2.4/site-packages/django/db/models/fields/related.py",
> line 244, in __init__
> raise ValueError("%r instance needs to have a primary key value
> before a many-to-many relationship can be used." % model)
> ValueError: <class 'django.contrib.auth.models.User'> instance needs to
> have a primary key value before a many-to-many relationship can be used.
When you create a many-to-many association, we store the pair of related
primary key values in the "join" table in the database. In order to
store that pair, both objects need to *have* a primary key. A new object
does not receive a primary key value until you save it (unless you
manually set the value yourself).
This will work:
o = Organization()
o.save()
o.sponser
Regards,
Malcolm
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