Please see wiki for discussion. The DM polymorphic gem mimics AR style polymorphism. It has been decided that DM will not follow this path, since it really isn't very nice on the DB and there are other ways, that do not require DBA to cry to achieve the same result. That being said this is still useful for those ppl wishing to convert rails apps over. One Massive Caveat is that using this you will get n+1 calls to the db atm if you do Comments#commentable for example. It needs a proxy object if anyone cares to write one. This is highly experimental software, use it at your own risk. ==== Example Usage. class Comment include DataMapper::Resource is :polymorphic, :commentable property :id, Integer, :serial => true property :text, String end class Post include DataMapper::Resource property :id, Integer, :serial => true property :name, String has n, :comments, :polymorphically => :commentable end class Article include DataMapper::Resource property :id, Integer, :serial => true property :name, String has n, :comments, :polymorphically => :commentable end This will then provide the following methods Comment#commentable Comment#post Comment#article Post#comments Article#comments What is needed is a Comment.commentables or something method so there is a nice proxy to avoid having all the loading issues.