Avoid the "Cannot call get_descendants on unsaved Category instances"

ValueError when adding categories in admin interface.

categories/base.py

@@ -108,10 +108,11 @@ def clean(self):
         # Validate Category Parent
         # Make sure the category doesn't set itself or any of its children as
         # its parent.
-        decendant_ids = self.instance.get_descendants().values_list('id', flat=True)
+
         if self.cleaned_data.get('parent', None) is None or self.instance.id is None:
             return self.cleaned_data
-        elif self.cleaned_data['parent'].id == self.instance.id:
+        decendant_ids = self.instance.get_descendants().values_list('id', flat=True)
+        if self.cleaned_data['parent'].id == self.instance.id:
             raise forms.ValidationError(_("You can't set the parent of the "
                                           "item to itself."))
         elif self.cleaned_data['parent'].id in decendant_ids: