private function assert_dirtifies($method)
{
$model = new Author();
$datetime = new ActiveRecord\DateTime();
$datetime->attribute_of($model, 'some_date');
$args = func_get_args();
array_shift($args);
call_user_func_array(array($datetime, $method), $args);
$this->assert_has_keys('some_date', $model->dirty_attributes());
}
