I had been using acts_as_tree together with acts_as_list to make ordered nested lists. Recently I replaced acts_as_tree with the better_nested_set plugin, a formidable replacement that is much more efficient in database queries (it pulls the whole tree from the database at once). The nested set is also ordered, so there's no need for acts_as_list.

Unfortunately better_nested_set lacks the list methods provided by acts_as_list. Here's a module that provides them. It treats each branch of a tree as its own list, thus these methods are scoped by the local root.

I haven't bothered to make this a plugin. Just create a file lib/nested_set_list.rb and paste this code:

module NestedSetList
  def first?
    parent.nil? or parent.lft==self.lft-1
  end
  def last?
    parent.nil? or parent.rgt==self.rgt+1
  end
  def higher_item
    list = self_and_siblings
    i = list.index(self)
    i==0 ? self : list[ i-1 ]
  end
  def lower_item
    list = self_and_siblings
    i = list.index(self)
    i==list.size-1 ? self : list[ i+1 ]
  end
  def move_higher
    move_to_left_of( higher_item ) if higher_item
  end
  def move_lower
    move_to_right_of( lower_item ) if lower_item
  end
  def move_to_top
    move_to_left_of( self_and_siblings.first )
  end
  def move_to_bottom
    move_to_right_of( self_and_siblings.last )
  end
end 

In your model, be sure to include it

  acts_as_nested_set

  include NestedSetList