dm belongs_to key problem

gistfile1.rb

require 'dm-core'
require 'dm-migrations'
require 'dm-validations'

DataMapper.setup :default, "sqlite::memory"
class User
  include DataMapper::Resource

  property :id, Serial
  has 1, :image
end

class Image
  include DataMapper::Resource
  belongs_to :user, key: true
end

DataMapper.finalize
DataMapper.auto_migrate!

image = Image.new user: User.new

image.valid? # false - User must not be blank
image.save   # true - works as expected