undefined method `__sort_option__' for :ancestry:Symbol

[nahankid]

When trying to view a tree in ActiveAdmin, this is throwing error:
undefined method `sort_option' for :ancestry:Symbol

This doesn't work with the combination: Mongoid, mongoid-ancestry and active_admin-sortable_tree
But works fine with ActiveRecord, Ancestry and active_admin-sortable_tree

Sample source code is at: https://github.com/nahankid/community