Associations without foreign keys???

Hi Folks,

I have a game where users get access to technologies. The technology
records aren't owned - so a normal foreign key doesn't make sense.

I want to tell rails not to bother with xxx_id, and just to use
conditions I set.

I can use finder_sql for a has_many
    base.has_many :superstructures,
                  :class_name => 'Technology',
                  :finder_sql => "select * from technologies where
category = 'Superstructure'" do
                    def allowed
                      find :all, :conditions => 'research_required <

but the 'allowed' addition doesn't work

For a has_one, finder_sql doesn't seem to be an allowed option.

What I'd like is :foreign_key => nil, then I could put my logic
in :conditions
-but that doesn't seem to work either - rails just guesses the default

Is there a way to do this without just defining class methods on