this should work

small_test.rb

require 'rubygems'
require 'mongoid'
require 'mongoid_acts_as_tree'

Mongoid.configure do |config|
  name = "control_development"
  host = "localhost"
  config.master = Mongo::Connection.new.db(name)
  config.persist_in_safe_mode = false
end


class Msg
  include Mongoid::Document
  include Mongoid::Acts::Tree
  acts_as_tree

	field :name
end

if __FILE__ == $PROGRAM_NAME
	require "test/unit"
	class TestMsg < Test::Unit::TestCase

		def test_move_children_with_children
			Msg.destroy_all

			parent1 = Msg.create! :name => 'name1'
			parent2 = Msg.create! :name => 'name2'

			fl_child = Msg.create! :name => 'test1', :parent_id => parent1.id
			sl_child = Msg.create! :name => 'test2', :parent_id => fl_child.id

			assert_equal ['test1', 'test2'], parent1.descendants.map {|o| o.name}
			assert_equal [], parent2.descendants.map {|o| o.name}

			a = Msg.find(:first, :conditions => {"name" => "test1"})
			a.parent_id = parent2.id
			a.save!

			assert_equal [], parent1.descendants.map {|o| o.name}
			assert_equal ['test1', 'test2'], parent2.descendants.map {|o| o.name}

			assert_equal ['test2'], a.children.map {|o| o.name}

		end

	end
end