从 basicdblist Mongo 中检索值

Question

{
DB db = Helper.connectingMonggo();
BasicDBObject query = new BasicDBObject();
query.put("_id", "123");
DBCollection table = new db.getCollection("prodDetail");
DBCursor cursor = table.find(query); --// cursor has only one record (size =1)

while(cursor.hasNext()){
  DBObject obj = cursor.next();
  List<String> upcs = new ArrayList<>();
  BasicDBList list = (BasicDBList) obj.get("prodList"); --//here prodList size also 1 always
  DBObject obj1 = (DBObject) list.get(0);
  BasicDBList detailsList = (BasicDBList) obj1.get("pDetailsList");

 for(Object obj2 : detailsList){
   JSONObject obj3 = new JSONObject(JSON.serialize(obj2));
   ups.add(obj3.get("upcStatus").toString());
 } } }

I am getting the list array with upcStatus but, is it proper way to write?

Answer 1

This is an aggregation query using the MongoDB Java driver v3.12 and MongoDB v4.2. Note the result is returned using a single query and no additional code. This is useful as the query is entirely run on the server and the result is returned to your Java application.

try(MongoClient mongoClient = MongoClients.create()) {
    MongoDatabase database = mongoClient.getDatabase("test");
    MongoCollection<Document> coll = database.getCollection("testCollection");
    List<Bson> pipeline = Arrays.asList(
        Aggregates.match(Filters.eq("_id", 123)),
        Aggregates.project(Projections.fields(
            Projections.excludeId(), 
                Projections.computed("upcStatus", Filters.eq("$arrayElemAt", 
                    Arrays.asList("$prodList.pDetailsList.upcStatus", 0L))
                )
        ))
    );
    
    Document doc = coll.aggregate(pipeline).first();
    System.out.println(doc.toJson());    // prints the array of upcStatus field values
}

The classes used in the code are defined as following classes or in packages: org.bson.Document , org.bson.conversions.Bson , com.mongodb.client.model.* and com.mongodb.client.* .

Also, note the try statement-block opens the connection to the server and closes automatically at the end of the try-block.