Get an object other than primary key ID

Hello,
         i want to retrieve and object from a table other than primary
key ID.

eg: @sites.each do |@site|
          @site_id = @site.id
          @tower = Tower.find(:all, :conditions => ["site_id =?",
@site_id])

Tower Belongs to Site. so Site_id is a reference in tower object.
there will be only one Tower object for Site Object.

Should we give relation as has_obne in site model??

help me ot.

Karthik