如何在环回模型查询中对嵌套对象应用字段过滤器？

Question

我在mongodb上的表中以geojson格式将数据存储在名为“ Alerts”的表中

{
 "_id" : ObjectId("5be40b8099d5b81e6115d517"),
"title" : "Real-time earthquake",
"priority" : "high",
"status" : NumberInt("1"),          
"extAlertData" : {
                 "type" : "Feature",
                  "properties" : {
                                  "mag" : 4.11,
                                  "place" : "12km S of Tres Pinos, CA",
                                  "time" : 1541163534230,
                                  },
                  "geometry" : {
                                 "type" : "Point",
                                 "coordinates" : [
                                                 -121.3146667,
                                                 36.6801667,
                                                 6.48
                                                 ]
                               },
                   "id" : "nc73105276"
                 }
}
{
"_id" : ObjectId("5be401c39b9e021a4c1a5c80"),
"title" : "Real-time earthquake",
"priority" : "high",
"status" : NumberInt("1"),          
"extAlertData" : {
                  "type" : "Feature",
                  "properties" : {
                                  "mag" : 4.5,
                                  "place" : "107km NW of Tobelo, 
                                             Indonesia",
                                  "time" : 1541665816710,
                                 },
                  "geometry" : {
                               "type" : "Point",
                               "coordinates" : [
                                               127.2763,
                                               2.3671,
                                               31.55
                                               ]
                               },
                   "id" : "us1000hp6t"
                 }
}

我想使用模型查询来检索嵌套在“ extAlertData”中的字段“ id”。

以下查询无效。

db.Alerts.find({fields : {"extAlertData.id": true}},
                        function (err, Alerts){
                            //process alert extAlertData.id
                        }
}); 

查询正在检索以下错误：

TypeError: Cannot read property 'id' of undefined

如何在mongoDB中使用字段过滤器进行查询？我正在使用节点js的回送框架。

先谢谢了。