class Person < ActiveRecord::Base
  belongs_to :country
  define_index
    indexes :name
    indexes country(:name), :as => country_name
  end
end

class Country < ActiveRecord::Base
  has_many :people # has_many :persons # depending on your singular/plural case
  define_index
    indexes :name
  end
end
	
ThinkingSphinx.search :conditions => {:name => params[:q]}, :classes => [Person, Country]
	
class Person < ActiveRecord::Base
  belongs_to :country
  define_index :my_first_in do
    indexes :name
    indexes country(:name)
  end
  define_index :my_second_in do
    indexes :name
  end
end