实体类A里引用了实体类B,并用了DBRef，现在想查询A的部分字段和包含的B里的部分字段出来，发现没办法做到，哪位大神有方法？代码如下：

public class A {

@Id
private int id;

@DBRef
private B b;

}

public class B{

@Id
private int id;

private String name;

}

Dao代码如下,
BasicDBObject queryObject = new BasicDBObject();
queryObject.put("id", id); //A的id
BasicDBObject fieldsObject=new BasicDBObject();
fieldsObject.put("b", 1); //可以拿到B的全部字段
//fieldsObject.put("b.name", 1); //亲测失败，报错是无法给B的id设置成null，未找到原因
Query query=new BasicQuery(queryObject,fieldsObject);
mongoOperation.find(query, A.class);