Spring Data MongoDB-MappedResults返回空白列表

Question

Mongodb查询为：

db.getCollection('country2').aggregate([{"$lookup":{"from":"state2","localFiel
        d":"_id","foreignField":"countryId","as":"state2"}}]).pretty()

返回此结果：

{
        "_id" : ObjectId("5b2b520333a04107677b5ce1"),
        "countryName" : "India",
        "state2" : [
                {
                        "_id" : ObjectId("5b2b515b33a04107677b5cdb"),
                        "stateName" : "UP",
                        "countryId" : ObjectId("5b2b520333a04107677b5ce1")
                },
                {
                        "_id" : ObjectId("5b2b516033a04107677b5cdc"),
                        "stateName" : "MP",
                        "countryId" : ObjectId("5b2b520333a04107677b5ce1")
                }
        ]
}

尽管原始结果包含预期结果，但聚合查询返回空白的MappedResults。 State2是我的Java Pojo。

public List<BasicDBObject> getAllData() {
        LookupOperation lookUpOprn = LookupOperation.newLookup()
                .from("state2")
                .localField("_id")
                .foreignField("countryId")
                .as("state2");

        org.springframework.data.mongodb.core.aggregation.AggregationOptions options = Aggregation.newAggregationOptions().cursor(new BasicDBObject()).build(); 
        Aggregation aggregation = Aggregation.newAggregation(lookUpOprn).withOptions(options);

        List<BasicDBObject> list =  mongoOperations.aggregate(aggregation, "country2", BasicDBObject.class).getMappedResults();
        return list;
    }

可以帮忙吗？