dm-polymorphic.

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.

  class Comment
    include DataMapper::Resource
  
    property :id,   Serial
    property :text, String

    belongs_to :commentable, :polymorphic => true
  
    # OR use the old syntax
    #
    # is :polymorphic, :commentable
    #
  end
    
  class Post
    include DataMapper::Resource
  
    property :id, Serial
    property :name,  String  

    has n, :comments, :as => :commentable

    # OR use the old syntax
    #
    # has n, :comments, :polymorphically => :commentable   
    #
  end

  class Article
    include DataMapper::Resource
  
    property :id, Serial
    property :name,  String  

    has n, :comments, :as => :commentable

    # OR use the old syntax
    #
    # has n, :comments, :polymorphically => :commentable   
    #
  end

This will then provide the following methods

  Comment#commentable
  Comment#post
  Comment#article
  Post#comments
  Article#comments

  article = Article.create
  post = Post.create
  c1 = Comment.new
  c2 = Comment.new
  post.comments << c1
  post.save
  article.comments << c2
  article.save

  c1.commentable    # => post
  c2.commentable    # => article

  c1.post           # => post
  c1.article        # => nil

  c2.post           # => nil
  c2.article        # => article

  post.comments     # => [c1]
  article.comments  # => [c2]

no n+1 problem anymore