Symfony2 合并用户提供器问题

目前学习测试用的 Symfony2 版本为 2.0.1，在测试过程中想合并用户提供器为

providers:                                                                  
    main:                                                                   
      users:                                                                
        xiean: { password: **************************, roles: 'ROLE_ADMIN' }
      entity:  { class: Passport\UserBundle\Entity\User, property: username }

结果用户 xiean 死活无法认证，最后想把用户提供器分开测试，即

providers:                                                                   
    chain_provider:                                                         
      providers: [in_memory, in_db]                                          
    in_memory:                                                              
      users:                                                                
        xiean: { password: **************************, roles: 'ROLE_ADMIN' }
    in_db:                                                                  
      entity:  { class: Passport\UserBundle\Entity\User, property: username }

则一切正常，而出错的使用方式来自于官方文档

http://symfony.com/doc/2.0/book/security.html#using-multiple-user-providers