class Piece
  include DataMapper::Resource

  property :id,         Integer, :serial => true
  property :name,       String
  property :size,       String
  property :medium,     String
  property :price,      Integer
  property :sold,       Boolean
  property :category,   String

  property :created_at, DateTime
  property :updated_at, DateTime

  validates_present :name

  before :save, :update_times

  def update_times
    created_at ||= DateTime.now
    updated_at = DateTime.now
  end

end

>> p = Piece.new
=> #
>> p.name = "Foobar"
=> "Foobar"
>> p.valid?
=> true
>> p.save
NoMethodError: super: no superclass method `save'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-validations-0.9.1/lib/dm-validations.rb:35:in `save'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.1/lib/data_mapper/hook.rb:112:in `call'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.1/lib/data_mapper/hook.rb:112:in `save'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.1/lib/data_mapper/hook.rb:108:in `catch'
	from /opt/local/lib/ruby/gems/1.8/gems/dm-core-0.9.1/lib/data_mapper/hook.rb:108:in `save'
	from (irb):9