Caio César Caio César - 1 month ago 22
Java Question

Get Average from Mongo Collection using Aggrerate

I am trying to learn how to use the Mongo in Java and have been able to make some simple queries but I have been having trouble with the aggregate operator.

The document structure is a simple one, as the following:

{
"_id" : ObjectId("57dbe94f0507a4d8710ac5b2"),
"name" : "Name1",
"age" : 23
}
{
"_id" : ObjectId("57dbe9750507a4d8710ac5b3"),
"name" : "Name2",
"age" : "String for examble"
}
{
"_id" : ObjectId("57dbee630507a4d8710ac5b5"),
"name" : "Name3",
"age" : 24
}


All I want to do is get the average of the
ages
in the collection ( name example ).

Simply using mongo I can get the desirable result with the following consult:

db.example.aggregate([
{
$group: {
_id: null,
averageAge: { $avg: "$age" }
}
}
]);


I have tried the following:

BasicDBObject groupFields = new BasicDBObject("_id", "null");
BasicDBObject media = new BasicDBObject("$avg", "$age");
groupFields.put("mediaIdade", media);
BasicDBObject group = new BasicDBObject("$group", groupFields );
AggregateIterable<org.bson.Document> agregate = db.getCollection("exemplo").aggregate(Arrays.asList (group));


Which is almost a direct translation but got a
"java.lang.NoSuchMethodError: org.bson.codecs.BsonTypeClassMap.keys()Ljava/util/Set;"
, unsurprisingly.

But I cannot translate that to Java. I have checked and found this question but could not understand it due to the use of opperators such as
$unwind
. So I'm trying to make query as simple as possible to better understand how the Java framework for aggregation works.

Can someone help?

Answer

Try something like this.

MongoCollection<Document> dbCollection = db.getCollection("example", Document.class);
AggregateIterable<org.bson.Document> aggregate = dbCollection.aggregate(Arrays.asList(Aggregates.group("_id", new BsonField("averageAge", new BsonDocument("$avg", new BsonString("$age"))))));
Document result = aggregate.first();
double age = result.getDouble("averageAge");

Input:

{ "_id" : ObjectId("58136d6ed96cc299c224d529"), "name" : "Name1", "age" : 23 }
{ "_id" : ObjectId("58136d6ed96cc299c224d52a"), "name" : "Name2", "age" : 26 }
{ "_id" : ObjectId("58136d6ed96cc299c224d52b"), "name" : "Name3", "age" : 24 }

Output:

24.333333333333332
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