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
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
on creation, set automatically lft and rgt to the end of the tree