关于thinkphp框架的关联查询 RealtionModel

Question

class UserModel extends RelationModel{
$_link = array{
'foreign_key'=>'deptId',
'class_name'=>'dept',
'as_fields'=>'dept_name,dept_phone',
'mapping_type'=>BELONGS_TO
}
}
User表中有deptId外键，
$U = new UserModel();
$U->relation(true)->where("username = 'Lucy'")->select();//可以起作用
$U->relation(true)->where("username = 'Lucy' and dept_phone = '123456'")->select();//这句的第二个条件就不能起作用了