Has_one with :source_type

http://www.javaeye.com/topic/129195

class Client < ActiveRecord::Base
  has_many :contact_cards
  has_many :business_contacts,
           :through => :contact_cards,
           :source => :contact,  <= 跟 as => :this_item  的this_item 一樣  
           :source_type => 'Business'  
end

class Business < ActiveRecord::Base
  has_many :contact_cards, :as => :contact
end

class ContactCard < ActiveRecord::Base
  belongs_to :client
  belongs_to :contact, :polymorphic => true
end

User.first.business_cards  <= 第一個user的contact card of bussiness category