<?php

namespace foo\bar\biz;

class User extends \ActiveRecord\Model {
    static $has_many = array(
        array('user_newsletters', 'class_name' => '\foo\bar\biz\UserNewsletter'),
        array('newsletters', 'class_name' => '\foo\bar\biz\Newsletter',
               'through' => 'user_newsletters')
    );
}
// …

$user->newsletters;
// ReflectionException: Class UserNewsletter does not exist