'Group by multiple fields using mongodb aggregate builders in java application

I am fetching data from mongodb and doing some operations using aggregates builders in my java application. I was able to group by single field using the below piece of code.

Bson group = group("$city", sum("totalPop", "$pop"));
Bson project = project(fields(excludeId(), include("totalPop"), computed("city", "$_id")));
List<Document> results = zips.aggregate(Arrays.asList(group, project)).into(new ArrayList<>());

Now I need to group by using multiple fields...say city and location. Can someone help on this?

javamongodbgroup-byprojectaggregates


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