mongo-db Aggregation query working on mongo shell but not in java program

Question

I am working on a project in which i have a huge collection in inside my mongo db. Now I have to get some details from the mongo db which are pretty much smiliar like SQL group by clause. I successfully executed the query on mongo shell :

    > db.votes.aggregate(
    ... { $group : {
    ... _id : "vote_post_id",
    ... votesPerId : { $sum : 1 }
    ... }}
    ... );
    {
"result" : [
            {
                    "_id" : "vote_post_id",
                    "votesPerId" : 27371750
            }
    ],
    "ok" : 1

    }
    >

Now while executing the query from a java proggram I am facing follwing error :

    Exception in thread "main" com.mongodb.CommandResult$CommandFailure: command 
    failed [aggregate]: { "serverUsed" : "localhost/127.0.0.1:27017" , 
    "errmsg" : "exception: aggregation result exceeds maximum document size (16MB)"
     , "code" : 16389 , "ok" : 0.0}
    at com.mongodb.CommandResult.getException(CommandResult.java:88)
    at com.mongodb.CommandResult.throwOnError(CommandResult.java:134)
    at com.mongodb.DBCollection.aggregate(DBCollection.java:1311)
    at main.Connetion.CheckConnection.GetNoOfVotesForAType(CheckConnection.java:210)
    at main.DAO.Votes.VoteDAO.findKeyWord(VoteDAO.java:109)
    at main.DAO.Votes.VoteDAO.main(VoteDAO.java:139)

I have debugged my java program and it is producing the exact query what I wrote on mongo shell :

    { "$group" : { "_id" : "$vote_post_id" , "count" : { "$sum" : 1}}}

I have been through a lot of answers for this problem and none seems to solve the issue.

My java code for creating the aggregation function :

// create our pipeline operations, first with the $match DBObject match = new BasicDBObject("$match", new BasicDBObject("vote_type_id", "1") );

    // build the $projection operation
    DBObject fields = new BasicDBObject("vote_post_id", 1);
    fields.put("_id", 0);
    DBObject project = new BasicDBObject("$project", fields );

    // Now the $group operation
    DBObject groupFields = new BasicDBObject( "_id", "$vote_post_id");
    groupFields.put("count", new BasicDBObject( "$sum", 1));
    DBObject group = new BasicDBObject("$group", groupFields);

    // run aggregation
    AggregationOutput output = votesCollection.aggregate( match, project, group );

    System.out.println(output.getCommandResult());

Answer 1

To make your query the same as in shell it would be:

// $group operation
DBObject groupFields = new BasicDBObject( "_id", "vote_post_id");
groupFields.put("votesPerId", new BasicDBObject( "$sum", 1));
DBObject group = new BasicDBObject("$group", groupFields);

// run aggregation
AggregationOutput output = votesCollection.aggregate( group );

System.out.println(output.getCommandResult());

Answer 2

In your shell query you're doing

_id : "vote_post_id",

but in your Java query you're doing

"_id", "$vote_post_id"

The difference is the dollar sign. $vote_post_id substitutes the value of that field to use as the bucket keys, whereas vote_post_id just uses that literal value (so there's just one bucket).