Gemfile

dm_version = '=0.10.2'
gem 'dm-core',               dm_version
gem "dm-aggregates",         dm_version
gem "dm-migrations",         dm_version
gem "dm-timestamps",         dm_version
gem "dm-types",              dm_version
gem "dm-validations",        dm_version

gem "data_objects",          '=0.10'
gem "do_sqlite3",            '=0.10'

validations.rb

#!/usr/bin/env ruby
require 'vendor/gems/environment'
Bundler.require_env

DataMapper.setup(:default, 'sqlite3:///tmp/bs.db')
class Sheep
  include DataMapper::Resource
  
  property :id, Serial
  property :name, String

  has n, :hooves
end

class Hoof
  include DataMapper::Resource

  property :id, Serial
  property :position, String

  belongs_to :sheep
end

DataMapper.auto_migrate!

s = Sheep.new(:name => 'Shaun')
h = s.hooves.new(:position => 'left-front')

h.valid?         # ==> false; the error is {:sheep_id=>["Sheep must not be blank"]}
h.save           # ==> true;  shouldn't this fail due to validations?
h.valid?         # ==> true;  so invalid -> save anyway -> valid? confused now.