code: def set_default_left_and_right
        maxright = nested_set_scope.remove_order_by.max(right_field_name) || 0
        self[left_field_name] = maxright + 1
        self[right_field_name] = maxright + 2
        self[:depth] = 0
      end
docs: on creation, set automatically lft and rgt to the end of the tree