has_many 的 source_type 可以是多个吗?
和 stackoverflow 上这位仁兄一样的疑问,但没看到回答 ruby on rails polymorphic has_many relationship - more than one :source_type? http://stackoverflow.com/questions/11458536/ruby-on-rails-polymorphic-has-many-relationship-more-than-one-source-type