has_many,through,source,source_type,foregin_key,class_name

  #我發表過forum的comments
  has_many :forum_comments, :through => :forums, :source => :comments

 
  #forum 裡有我的 comments的 forum, 找出 forum array
  has_many :forums_has_comments,
           :through => :comments,
           :source  => :record,
           :source_type => 'Forum'

  #我在forum 裡所有的comments, 找出 comments array
  has_many :comments_in_forums,
           :foreign_key => :user_id,
           :class_name => 'Comment',
           :conditions => {:record_type => 'Forum'}