gistfile1.rb

require 'rubygems'
#gem "dm-core", "=0.9.7" #=> error!
#gem "dm-core", "=0.9.8" #=> error!
require 'dm-core'

DataMapper.setup(
  :default,
  "sqlite3:///#{File.expand_path(File.dirname(__FILE__))}/test.db"
)

class User
  include DataMapper::Resource
  property :id, Integer, :serial => true
  property :key, String
  has n, :items
end

class Item
  include DataMapper::Resource
  property :id, Integer, :serial => true
  belongs_to :user
end

DataMapper.auto_migrate!

user = User.create(:key => "key")
user.items << Item.create # ここでエラーになる
user.save