Mongo aggregation in java: group with multiple fields

Finally I've come to a solution. There were some errors in the question that I posted, as it was the last attempt after reaching some point of desperation, but finally, here is the final solution:

MongoDatabase mongo = // initialize your connection;Document matches = new Document("$match",    new Document("gi", new Document("$ne", null))    .append("ci", Integer.parseInt(courseid)));Document firstGroup = new Document("$group",    new Document("_id",        new Document("ci", "$ci")        .append("gi", "$gi")        .append("gn", "$gn")        .append("si", "$si"))    .append("count", new Document("$sum", 1)));Document secondGroup = new Document("$group",    new Document("_id",        new Document("ci", "$_id.ci")        .append("gi", "$_id.gi")        .append("gn", "$_id.gn"))    .append("ns", new Document("$sum", 1))); Document sort = new Document("$sort",     new Document("_id.gi", 1));             List<Document> pipeline = Arrays.asList(matches, firstGroup,    secondGroup, sort);AggregateIterable<Document> cursor = mongo.getCollection("I1")    .aggregate(pipeline);for(Document doc : cursor) { // do stuff with doc }

Instead of trying to create lists of key-values, I just appended the elements to the documents. Hope it will be useful for somebody!!

This question is quite old but was the top match on google when I searched so if anyone is looking for a solution to this I managed to do it in the following way

Aggregation.group(Fields.fields()            .and("field1")            .and("field2"))            .first("name")            .`as`("name")            .count().`as`("count")

This will produce the following MDB query:

   { "$group" :         { "_id" :         { "field1" : "$field1", "field2" : "$field2"},         "name" : { "$first" : "$name"},         "count" : { "$sum" : 1}   }