can I use acts_as_list with a has_many :through association

I'd like to be able to use a has_many :through association and treat the associations as a list but I'm getting this error when I try an use an acts_as_list method:

   NoMethodError: undefined method `move_to_bottom'

I'm using edge rails r6786.

Here are my domain rules:

   Activities are things students can do.
   Units consists of a sequenced list of Activities.
   Activities can be used in many Units.

Here are the associations for these rules:

class Activity < ActiveRecord::Base
   has_many :unit_activities
   has_many :units, :through => :unit_activities
end

class Unit < ActiveRecord::Base
   has_many :unit_activities, :order => :position
   has_many :activities, :through => :unit_activities
end

class UnitActivity < ActiveRecord::Base
   belongs_to :activity
   belongs_to :unit
   acts_as_list :scope => :unit_id
end

In he example below in script/console I load in some previously created activities; make a new unit; and add the activities to the unit.

activity_crystals = Activity.find_by_name("Molecular crystals")
activity_browning = Activity.find_by_name("Brownian motion")
activity_mixing = Activity.find_by_name("Temperature of mixing water - TEEMSS demo")
my_unit = Unit.create(:name => "My First Unit", :description => "A simple test unit to see if thenew associations work")

my_unit.activities.push(activity_crystals, activity_browning, activity_mixing)

my_unit.activities.each { |a| puts "#{a.id}: #{a.name}" }; nil

55: Molecular crystals
42: Brownian motion
25: Temperature of mixing water - TEEMSS demo

Here's where the error occurs:

my_unit.activities[0].move_to_bottom

NoMethodError: undefined method `move_to_bottom' for #<Activity:0x34dec54>
         from

At this point my unit_activities table looks like this:

   id activity_id unit_id position

my_unit.activities accesses the items of the Activities table, which
are not marked as acts_as_list
The unit_activities items are enabled as acts_as_list.
Unit -> unit_activites -------> activties
           (acts_as_list) (NOT acts_as_list)

by adding these activities to your association, a new unit_activities
item was created for each activity. and those are acts_as_list

try this:

my_unit.unit_activities[0].move_to_bottom

my_unit.activities accesses the items of the Activities table, which
are not marked as acts_as_list
The unit_activities items are enabled as acts_as_list.
Unit -> unit_activites -------> activties
          (acts_as_list) (NOT acts_as_list)

by adding these activities to your association, a new unit_activities
item was created for each activity. and those are acts_as_list

try this:

my_unit.unit_activities[0].move_to_bottom

Thanks for taking the time to write Thorsten. That makes everything MUCH more clear!

Had problems with this too. Something you might want to have a look at
is this:
http://euphemize.net/blog/archives/2007/06/04/learning-rails-many-to-many-relationships/

Finally got things working.