Belongs_to field with shared foreign key not showing in form

[Jamevidi]

I have two belongs_to associations sharing the same foreign key (page_id).

 belongs_to :page
 belongs_to :special_page, :class_name => "SpecialPage", :foreign_key => "page_id"

this is the example form:

edit do 
  field :special_page
end

....the special_page field is not showing up in the form.

Is it a bug or made so by design? or something I missed? Its version 0.6.6.

Many thanks for any input! :-)