Java MongoDB aggreate query

Question

I need guidance in translating the following MongoDB query in to a Java program that uses a Java DB driver to retrieve data. Any guidance would be really appreciated.

db.playerscorecollection.aggregate([ 

{ 

 $unwind: "$scorearray"

}, 

{ 
$group: 
{ 

   _id: {player:'$player,venue:'$venue'}, maxScore: { $max: "$scorearray.score" } 

} 

} 

]);

Actually I got it wrong, the mongodb query that I posted in the question is grouping the results based on player and venue and then caluclating the max score , I need to get the venue for the max score only , if I take venue out of the id field I get an error , can someone help? — user1965449
– user1965449, Commented Jul 13, 2013 at 4:43
You need to learn to say what error you get! In any case, you can change the _id part to: _id : '$venue' — but of course it doesn't return players then. — Derick
– Derick, Commented Jul 13, 2013 at 15:26

Derick · Accepted Answer · 2013-07-11 07:38:46Z

1

The documentation and tutorial at http://docs.mongodb.org/ecosystem/tutorial/use-aggregation-framework-with-java-driver/ is quite extensive. Please read that first before reading the remainder of this answer. In general, something like this should work:

// create our pipeline operations, first the $unwind
DBObject unwind = new BasicDBObject( "$unwind", "$scorearray" );

DBObject groupIdFields = new BasicDBObject( "player", "$player" );
groupIdFields.put( "venue", "$venue" );
DBObject groupFields = new BasicDBObject( "_id", groupIdFields );
groupFields.put( "maxScore", new BasicDBObject( "$max", "$scorearray.score" ) );
DBObject group = new BasicDBObject( "$group", groupFields );

// run aggregation
AggregationOutput output = MDB.getCollection("playerscorecollection").aggregate( unwind, group );

answered Jul 11, 2013 at 7:38

Derick

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5 Comments

user1965449 Over a year ago

Thank You ! That was accurate.

user1965449 Over a year ago

Thank You , how can I add distinct to it. ?

user1965449 Over a year ago

Actually I got it wrong, the mongodb query that I posted in the question is grouping the results based on player and venue and then caluclating the max score , I need to get the venue for the max score only , if I take venue out of the id field I get an error , can someone help?

Derick Over a year ago

You need to learn to say what error you get! In any case, you can change the _id part to: _id : '$venue' — but of course it doesn't return players then

user64141 Over a year ago

Anybody know what happened to the page t docs.mongodb.org/ecosystem/tutorial/… It's not there anymore. Is there an update somewhere with the new API in version 3?

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