association extension method

http://blog.hasmanythrough.com/2006/8/19/magic-join-model-creation

  has_many :targets, :foreign_key =>"target_type_id" do
    def stock
      find(:all, :conditions => 'target_cat_id = "STO" and status <> 0 and target_class <> "0"' )
    end
  end

其實可以直接在target model 建 self.stock 的method 也可以用, 我還不知道差在哪 @@