Question

i’m developing a authentication backend with object-based permissions for my django-app.I use generic relations between an object and a permission:

class GroupPermission(models.Model):
    content_t= models.ForeignKey(ContentType,related_name='g_content_t')
    object_id = models.PositiveIntegerField()
    content_object = generic.GenericForeignKey('content_t', 'object_id')
    permission=models.ForeignKey(Permission)
    group = models.ForeignKey(Group)

And now i want to get all objects of a specified content type for which a сertain group or user has a сertain permission. What’s the best way to do that?
Shall i define the second end of the relation in the app’s models? or better write custom sql?
I’m trying to build a generic backend so i don’t want it to depend on the app that is using it.
Thanks!

Answer 1

I’m guessing you’re looking for something like:

perm = Permission.objects.get(pk=1) # pk #1 for brevity.
group = Group.objects.get(pk=1)     # Again, for brevity.
group_perms = GroupPermission.objects.filter(permission=perm, group=group)
objects = [x.content_object for x in group_perms]

This should get all of the objects which have the Permission of perm, and the Group of group into the variable objects.

You could implement this into a Custom Manager class, as well:

class GroupPermissionManager(models.Manager):
    def for(self, perm):
        group_perms = GroupPermission.objects.filter(permission=perm, group=self)
        objects = [x.content_object for x in group_perms]

class Group(models.Model):
    name = models.CharField(max_length=30)
    permissions = GroupPermissionManager()

Which would make your view code simpler:

perm = Permission.objects.get(pk=1) # pk #1 for brevity.
group = Group.objects.get(pk=1)     # Again, for brevity.
objects = group.permissions.for(perm)