def add_perm(self, perm):
        if self.has_perm(perm):
            return
        
        if perm in _custom_permissions:
            p = _custom_permissions[perm]
            
            if hasattr(p, 'add_permission'):
                p.add_permission(user)
                self._cache_perm(perm)
        else:
            try:
                p = Permission.objects.get(codename=perm)
                self.permissions.add(p)
                self._cache_perm(perm)
            except Permission.DoesNotExist:
                pass