Retrive a set of documents from array in MongoDB using Java driver

Question

I have a MongoDB document structure like this:

{ 
  "name": "list"
  "config": "default"
  "items": [
    {
      "email": "user1@mail.com"
      "status": true
      "number": 123
    },
    ...
    {
      "email": "user100@mail.com"
      "status": false
      "number": 432
    },
   ]
}

Now, how can I retrive multiple subdocuments that much certain criteria. For instance, I want to obtain all documents where status = true. I know that it is possible to use $elemMatch, but it returns only the first matching instance and I need ALL documents that correspond to the specified condition. Please show how can it be done with Java.

Answer 1

You can do it in Java as follows.

        Mongo mongo = new Mongo("localhost:27017");
        DB db = mongo.getDB("myDB");
        DBCollection coll = db.getCollection("myCollection");

        DBObject statusQuery = new BasicDBObject("status", true);
        DBObject elemMatchQuery = new BasicDBObject("$elemMatch", statusQuery);

        DBObject fields = new BasicDBObject();
        fields.put("items", elemMatchQuery);
        fields.put("name", 1);
        fields.put("config", 1);

        DBCursor cur = coll.find(new BasicDBObject(), fields);

        while (cur.hasNext()) {
            DBObject obj = cur.next();
            // Get fields from object
        }

Answer 2

If you want all subdocuments from the list items in separate documents, you can $unwind the list. I also added filter matching status is true:

try (MongoClient client = new MongoClient()) {

        MongoDatabase db = client.getDatabase("myDB");
        MongoCollection<BsonDocument> collection = db.getCollection("myColl", BsonDocument.class);

        MongoCursor<BsonDocument> cursor =
                collection.aggregate(Arrays.asList(new Document("$unwind", "$items"),
                                                    new Document("$match", new Document("items.status", true)))).iterator();

        try {
            while (cursor.hasNext()) {
                // I think this is what you need
                BsonDocument bsonDocument = cursor.next().getDocument("items");

                // and if you just want emails
                String email = bsonDocument.get("email").asString().getValue();
                System.out.println(email);
            }

        } finally {
            cursor.close();
        }
    }