nested_attrs_test.rb

require 'rubygems'
require 'dm-core'
require 'dm-postgres-adapter'
require 'dm-migrations'
require 'dm-validations'
require 'dm-transactions'

DataMapper::Logger.new($stdout, :debug)
DataMapper.setup(:default, 'postgres://user@localhost/nested_attrs_test')

class User
  include DataMapper::Resource

  property :id, Serial
  property :email, String

  has n, :handles
end

class Handle
  include DataMapper::Resource

  property :id, Serial
  property :name, String, :required => true

  belongs_to :user
end

DataMapper.finalize
DataMapper.auto_migrate!

u = User.new(:email => 'user@example.com', :handles => [Handle.new])
puts u.save # => false, but user is created, handle is not

puts u.handles.last.errors.inspect