gistfile1.rb

module MongoMapper
  module Associations
    class InArrayProxy < Collection
      def set_parent(parent, options={})
        self.each do |k| 
          k.set_parent(parent) 
          if (!options.has_key?(:recursive) or options[:recursive] == true) and k.send( reflection.name ).size > 0
            k.send( reflection.name ).set_parent(k, options)
          end
        end
      end
    end
  end
end

class Node
  include MongoMapper::Document

  key :ord, Integer, :index => true
  key :node_ids, Array, :index => true
  many :nodes, :in => :node_ids, :order => 'ord'

  def parent
    @parent
  end

  def siblings(node_id = nil)
    return [] if !parent
    if node_id
      node_id = Mongo::ObjectID.from_string(node_id) if node_id.is_a? String
      parent.nodes.each {|n| return n if n.id == node_id }
    else
      parent.nodes.reject{ |n| n == self }
    end
  end

  protected
    def set_parent(parent_node)
      @parent = parent_node
    end
end

n = Node.find(:first)
# sets 'n' as the @parent var on each child node, so the siblings method works properly
n.nodes.set_parent(n)