CakePHP 2：多个字段的身份验证（电子邮件或电话号码）

I have authentification system that allow login by email or phone number. How I can write custom scope, that will set username field to email if it contain @ and if not to phone ?

'authenticate' => array(
    'FormExtended' => array(
        'fields' => array(
            'username' => 'email',
            'password' => 'password',
        ),
        'scope' => array(
            'User.active' => true,
        ),
    )
)

Can this be done with scope condition ?