backtrace

/usr/lib/ruby/gems/1.8/gems/dm-core-0.9.8/lib/dm-core/adapters/data_objects_adapter.rb:79:in `execute_reader': ambiguous column name: articles.id (Sqlite3Error)
Query: SELECT "articles"."id", "articles"."name" FROM "articles" INNER JOIN "articles" ON "articles"."id" = "comments"."commentable_id" WHERE "articles"."id" = '3' AND "comments"."commentable_class" = 'Article' ORDER BY "articles"."id" LIMIT 1   from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.8/lib/dm-core/adapters/data_objects_adapter.rb:79:in `read_one'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.8/lib/dm-core/adapters/data_objects_adapter.rb:202:in `with_connection'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.8/lib/dm-core/adapters/data_objects_adapter.rb:74:in `read_one'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.8/lib/dm-core/repository.rb:131:in `read_one'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.8/lib/dm-core/model.rb:339:in `first'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.8/lib/dm-core/associations/many_to_one.rb:45:in `article_helper'
        from /usr/lib/ruby/gems/1.8/gems/dm-core-0.9.8/lib/dm-core/associations/many_to_one.rb:70:in `article'
        from lib/tester.rb:56

error.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], Comment.commentable_class => "Article"
end

DataMapper.auto_migrate!

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

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

puts comment.article.inspect

good.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"

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

puts comment.article.inspect