django foreign key on meta permission

Question

I have the following model in Django:

class Foo(models.Model):
    name = models.TextField()
    permission = models.ForeignKey(Permission, null=True, blank=True)

    class Meta:
        permissions = (
            ('permA', 'permA only for alpha-users'),
            ('permB', 'permB only for beta-users'),
        )

The problem is, that the permission-attribute in the class Foo could be any permission. But I would only allow the permissions which I have defined in the Meta class. So 'permA' and 'permB'. Can someone help me with this?

Thank you!