Rails - Jun 2006 - Reaching through a belongs_to for acts_as_list scope?

I''m running into a situation where I''d like to provide the
list scope
for an acts_as_list model from one step removed. In minimalistic form  
(with several other associations removed from each level).

class MainEntity < AcrtiveRecord::Base
   has_many :containers
end

class Container < ActiveRecord::Base
   has_many :elements
   belongs_to :main_entity
end

class Elements < ActiveRecord::Base
   belong_to :containers
   acts_as_list :scope=>  ???  # it wants to be container.main_entity_id
end

Now I''ve thought of two ways to solve this.
   1.  add main_entity_id to the Element -- this adds a redundant  
link in the DB, and I''d like to avoid that, but if this is the normal  
Rails solution, I guess I can live with it...

  2. Something like
     acts_as_list :scope=>''container_id IN (# 
{container.main_entity.containers.map { |c|
c.id}.join(",")})''

appears to work, but also appears a little cumbersome.

Is there a better way?

Eric