Get an object other than primary key ID

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

eg: @sites.each do |@site|
@site_id =
@tower = Tower.find(:all, :conditions => [“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.