changing parent_id does not update item positions

[richardrails]

I have this tree:

parent_id: 1

• item A position 1
• item B position 2
• item C position 3

parent_id: 2

• item D position 1
• item E position 2
• item F position 3

when I change parent_id attribute at item A: parent_id = 2 and save, it does not recalculate item positions of all lists! I get this lists after save:

parent_id: 1

• item B position 2
• item C position 3

parent_id: 2

• item A position 1
• item D position 1
• item E position 2
• item F position 3

and correct lists are:

parent_id: 1

• item B position 1
• item C position 2

parent_id: 2

• item A position 1
• item D position 2
• item E position 3
• item F position 4

Which method should I call to do this right order? Thank you.

[richardrails]

module ActsAsList
  # positions can get mixed up when users click like crazy -> reorder them if necessary
  # ActsAsList.reorder_positions!(current_user.categories)
  def self.reorder_positions!(objects)
    objects.each_with_index do |object, index|
      new_position = index + 1
      next if object.position == new_position
      object.update_attribute(:position, new_position)
    end
  end
end

  after_save :reorder_list, if: :parentid_changed?
  def reorder_list
    ActsAsList.reorder_positions!(Page.where(parentid: self.parentid).order(:position))
    ActsAsList.reorder_positions!(Page.where(parentid: self.parentid_was).order(:position))
  end