backtrace

/usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/relationship.rb:106:in `send': undefined method `comments_association' for nil:NilClass (NoMethodError)
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/relationship.rb:106:in `get_children'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/relationship.rb:105:in `each'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/relationship.rb:105:in `get_children'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/relationship.rb:172:in `with_repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/support/kernel.rb:6:in `repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core.rb:181:in `repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/repository.rb:44:in `scope'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core.rb:181:in `repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/support/kernel.rb:6:in `repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/relationship.rb:172:in `with_repository'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/relationship.rb:80:in `get_children'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/one_to_many.rb:79:in `all'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.9/lib/dm-core/associations/one_to_many.rb:17:in `comments'
        from test.rb:32

test.rb

require 'rubygems'
require 'dm-core'

DataMapper.setup(:default, 'sqlite3::memory:')

class Article
  include DataMapper::Resource

  property    :id,      Serial
  property    :title,   String

  has n, :comments, :child_key => [:commentable_id], :commentable_class => "Article"
end

class Comment
  include DataMapper::Resource

  property    :id, Serial
  property    :text, Text
  property    :commentable_class, String
  property    :commentable_id, String

  belongs_to :article, :child_key => [:commentable_id], :commentable_class => "Article"
end

DataMapper.auto_migrate!

article = Article.create :title => "Article"

article.comments.create(:text => "Comment")

puts article.comments(:text => "Comment").inspect