$cond operator java code

问题

I have an aggregation query as follows :

db.TWITTER_DATA_Processed.aggregate( {$match : { SpId : 840, Scheduler_id : "SCH_01" }},{$group : {_id : {SpId : "$SpId", Scheduler_id : "$Scheduler_id",Country : "$Country"}, positive_count :  { $sum: { $cond: [ { $gt: [ "$Sentiment", 0 ] } , 1, 0 ] } }, neutral_count :  { $sum: { $cond: [ { $eq: [ "$Sentiment", 0 ] } , 1, 0 ] } }, negative_count :  { $sum: { $cond: [ { $lt: [ "$Sentiment", 0 ] } , 1, 0 ] } }}})

Sample output :

 {
         "_id" : {
                 "SpId" : 840,
                 "Scheduler_id" : "SCH_01",
                 "Country" : "Argentina"
         },
         "positive_count" : 1,
         "neutral_count" : 2,
         "negative_count" : 0
 },
 {
         "_id" : {
                 "SpId" : 840,
                 "Scheduler_id" : "SCH_01",
                 "Country" : "United Kingdom"
         },
         "positive_count" : 17,
         "neutral_count" : 4,
         "negative_count" : 1
 }

I'm facing difficulty in writing the java code for the same because of the $cond operator. From the existing threads, I assume the $gt has to be written as follows :

DBObject positiveSentiment = new BasicDBObject("$Sentiment", new BasicDBObject("$gt", 0));

The only thing I could figure out about $cond was that it requires an operand array(during my experiments, I got the following error :

"errmsg" : "exception: the $cond operator requires an array of 3 operands" , "code" : 16019

But how do I fit

[ { $gt: [ "$Sentiment", 0 ] } , 1, 0 ]

in the Java code ?

Adding the final working code, just in case anyone needs in future for reference :

DBObject groupIdKeys = new BasicDBObject("SpId", "$SpId");
        groupIdKeys.put("Scheduler_id", "$Scheduler_id");
        groupIdKeys.put("Country", "$Country");

        /* positive sentiments */
        List<Object> gtOperands = new ArrayList<Object>();
        gtOperands.add("$Sentiment");
        gtOperands.add(0);

        List<Object> positiveCondOperands = new ArrayList<Object>();
        positiveCondOperands.add(new BasicDBObject("$gt", gtOperands));
        positiveCondOperands.add(1);
        positiveCondOperands.add(0);
        BasicDBObject finalPositiveCond = new BasicDBObject("$cond",
                positiveCondOperands);

        DBObject sumOfPositiveCount = new BasicDBObject("$sum",
                finalPositiveCond);

        /* neutral sentiments */
        List<Object> eqOperands = new ArrayList<Object>();
        eqOperands.add("$Sentiment");
        eqOperands.add(0);

        List<Object> neutralCondOperands = new ArrayList<Object>();
        neutralCondOperands.add(new BasicDBObject("$eq", eqOperands));
        neutralCondOperands.add(1);
        neutralCondOperands.add(0);
        BasicDBObject finalNeutralCond = new BasicDBObject("$cond",
                neutralCondOperands);

        DBObject sumOfNeutralCount = new BasicDBObject("$sum", finalNeutralCond);

        /* negative sentiments */
        List<Object> ltOperands = new ArrayList<Object>();
        ltOperands.add("$Sentiment");
        ltOperands.add(0);

        List<Object> negativeCondOperands = new ArrayList<Object>();
        negativeCondOperands.add(new BasicDBObject("$lt", ltOperands));
        negativeCondOperands.add(1);
        negativeCondOperands.add(0);
        BasicDBObject finalNegativeCond = new BasicDBObject("$cond",
                negativeCondOperands);

        DBObject sumOfNegativeCount = new BasicDBObject("$sum",
                finalNegativeCond);

        DBObject groupKeys = new BasicDBObject("_id", groupIdKeys);
        groupKeys.put("positive_count", sumOfPositiveCount);
        groupKeys.put("neutral_count", sumOfNeutralCount);
        groupKeys.put("negative_count", sumOfNegativeCount);

        DBObject group = new BasicDBObject("$group", groupKeys);

        AggregationOutput output = dbCollection.aggregate(firstOp, group);

        Iterable<DBObject> results = output.results();

        for (DBObject result : results) {
            System.out.println(result);
        }

回答1:

Following example here you can create the array for "$cond" operator like this:

ArrayList gtA = new ArrayList();
gtA.add("$Sentiment");
gtA.add(0);
ArrayList condArray = new ArrayList();
condArray.add(new BasicDBObject("$gt", gtA));
condArray.add(1);
condArray.add(0);
BasicDBObject fullCond = new BasicDBObject("$cond", condArray);

来源：https://stackoverflow.com/questions/20606089/cond-operator-java-code