在使用java获取文档时，mongodb在mongo查询中传递DBRef

Question

 db.collectionB.findOne()
 {
    "_id" : NumberLong(24),
    "class" : "Top",
    "type" : DBRef("collectionA", NumberLong(47))
 }
db.collectionA.findOne()
{
   "_id" : NumberLong(47),
   "name" : "John",
   "position" : 2 
}

要形成的查询：db。收藏B。find（{type:DBRef（“collectionA”，NumberLong（47））}）

收藏a&；收藏B。我想搜索collectionB中的“type”指的是collectionA中“_id”是number long（47）的文档

BasicDBObject query =   new BasicDBObject("name","John");
DBObject db_object = findOne("collectionA",query);
DBRef myDbRef = new DBRef(db,"collectionB",db_object); 
DBObject doc = myDbRef.fetch(); 
System.out.println(doc);

它给出的输出为空。为什么