has_many_polymorphs and acts_as_list ?

First, BRAVO for this wonderful plugin: has_many_polymorphs ! Now my
problem:

4 models : Collection, Page, Fragment, Belonging (which is the join
table):

  1 class Collection <
ActiveRecord::Base
  2 has_many_polymorphs :elements,
  3 :through => :belongings,
  4 :from => [:pages, :fragments, :collections],
  5 :as => :collector,
  6 :parent_order => 'belongings.position'
  7 end

  1 class Page <
ActiveRecord::Base
  2 has_many :fragments
  3 end

  1 class Fragment <
ActiveRecord::Base
  2 belongs_to :page
  3 end

  1 class Belonging <
ActiveRecord::Base
  2 belongs_to :collector,
  3 :class_name => "Collection",
  4 :foreign_key => "collector_id"
  5 belongs_to :element, :polymorphic => true
  6 acts_as_list :scope => 'element_id=#{element_id} and
element_type=#{quote_value element_type}'
  7 end

The belongings database table structure:

collector_id (integer) ; element_id (integer) ; element_type
(string) ; position (integer)

The self referential polymorphic aspect works like a charm!
But the acts_as_list doesn't!
Here a demonstration inside the console:

c = Collection.create(:name => "C1")
c.elements << Page.create(:name => "P1")
c.elements << Page.create(:name => "P2")
y c

--- &id006 !ruby/object:Collection
attributes:
  name: C1
  updated_at: &id001 2007-11-14 13:17:45.877143 +01:00
  id: 1
  created_at: *id001
belongings:
- !ruby/object:Belonging
  attributes:
    updated_at: 2007-11-14 13:18:39
    element_id: "1"
    id: "1"
    collector_id: "1"
    element_type: Page
    position: "1"
    created_at: 2007-11-14 13:18:39
- !ruby/object:Belonging
  attributes:
    updated_at: 2007-11-14 13:19:06
    element_id: "2"
    id: "2"
    collector_id: "1"
    element_type: Page
    position: "1"
    created_at: 2007-11-14 13:19:06
elements:
- &id003 !ruby/object:Page
  attributes:
    name: P1
    updated_at: &id002 2007-11-14 13:18:39.397292 +01:00
    id: 1
    created_at: *id002
  errors: !ruby/object:ActiveRecord::Errors
    base: *id003
    errors: {}

  new_record: false
  new_record_before_save: true
- &id005 !ruby/object:Page
  attributes:
    name: P2
    updated_at: &id004 2007-11-14 13:19:06.052972 +01:00
    id: 2
    created_at: *id004
  errors: !ruby/object:ActiveRecord::Errors
    base: *id005
    errors: {}

  new_record: false
  new_record_before_save: true
errors: !ruby/object:ActiveRecord::Errors
  base: *id006
  errors: {}

new_record: false
new_record_before_save: true
=> nil

c.elements.first.move_to_bottom

NoMethodError: undefined method `move_to_bottom' for #<Page:0x3318ba4>
        from /opt/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/
active_record/base.rb:1860:in `method_missing'
        from (irb):8

I would be very pleased if you could help me ...

Just for completness ...
In fact I have to refer to the belongings table when interested by the
acts_as_list behavior. So:
c.belongings.first.move_to_bottom
And not:
c.elements.first.move_to_bottom
etc. etc.

And in my case the scope I meant wasn't:
element_id=#{element_id} and element_type=#{quote_value
element_type}'
But:
collector_id
Since I want the elements inside a collection to act as a list.