How to aggregate values in an array of objects in MongoDB - mongodb

Question

I store documents from cars and want to get the temperature of all Mercedes cars as an array, how should the query be in Mongodb?
{ "_id" : { "$oid" : "5880ff305d15f416c89457b7" },
"car" : "mercedes",
"engine" : {
"sensor" : {
"temperatur" : "20",
"speed" : "100",
"hue" : "40"
}
},
"motor" : {
"Power" : "155",
"Topspeed" : "400"
}
}
{ "_id" : { "$oid" : "5880ff305d15f416c89457b7" },
"car" : "mercedes",
"engine" : {
"sensor" : {
"temperatur" : "50",
"speed" : "100",
"hue" : "40"
}
},
"motor" : {
"Power" : "155",
"Topspeed" : "400"
}
}
I would like to select the temperature for all Mercedes cars and receive it.
result should be like [20,50]
EDIT:
My code lookls like the following iam using JAVA:
MongoClient mongoClient = new MongoClient();
MongoDatabase database = mongoClient.getDatabase("test");
MongoCollection<Document> coll = database.getCollection("myTestCollection");

Answer 1

You can try something like this with regular query if you're okay with distinct values.
db.cars.distinct("engine.sensor.temperatur", {"car" : "mercedes"});
This will give you
[ "20", "50" ]
Update - Java equivalent:
List<String> temps = coll.distinct("engine.sensor.temperatur", new Document("car", "mercedes"), String.class).into(new ArrayList<>());
Update - Aggregation Option
Bson match = new Document("$match", new Document("car", "mercedes"));
Bson group = new Document("$group", new Document("_id", "$car").append("temps", new Document("$push", "$engine.sensor.temperatur")));
List<String> temps = (List<String>) coll.aggregate(Arrays.asList(match, group)).map(document -> document.get("temps")).first();