How do I implement this MongoDB aggregation in Java

Question

I have the following working MongoDB aggregation shell command:

db.followrequests.aggregate([{
    $match: {
        _id: ObjectId("551e78c6de5150da91c78ab9")
    }
}, {
    $unwind: "$requests"
}, {
    $group: {
        _id: "$_id",
        count: {
            $sum: 1
        }
    }
}]);

Which returns:

{ "_id" : ObjectId("551e78c6de5150da91c78ab9"), "count" : 7 }

I need to implement this in Java, I am trying the following:

List<DBObject> aggregationInput = new ArrayList<DBObject>();

BasicDBObject match = new BasicDBObject();
match.put("$match", new BasicDBObject().put("_id",new ObjectId(clientId)));
aggregationInput.add(match);

BasicDBObject unwind = new BasicDBObject();
unwind.put("$unwind", "$requests");
aggregationInput.add(unwind);

BasicDBObject groupVal = new BasicDBObject();
groupVal.put("_id", "$_id");
groupVal.put("count", new BasicDBObject().put("$sum", 1));

BasicDBObject group = new BasicDBObject();
group.put("$group", groupVal);
aggregationInput.add(group);

AggregationOutput output = followRequestsCol.aggregate(aggregationInput);
for (DBObject result : output.results()) {
    System.out.println(result);
}

I am getting an exception:

mongodb the match filter must be an expression in an object.

Can you please help me identify the error in the above code. Thanks!

Nicolas · Accepted Answer · 2016-01-10 15:42:34Z

Try to print the value of aggregationInput and you will realise that .put() does not return a BasicDBObject but just the previous value associated to the key you update. Therefore, when you do:

match.put("$match", new BasicDBObject().put("_id",new ObjectId(clientId)));

You are actually setting $match to null, as new BasicDBObject().put("_id",new ObjectId(clientId)) returns null.

Update you code to something like:

List <DBObject> aggregationInput = new ArrayList <DBObject> ();

BasicDBObject match = new BasicDBObject();
BasicDBObject matchQuery = new BasicDBObject();
matchQuery.put("_id", new ObjectId());
match.put("$match", matchQuery);
aggregationInput.add(match);

BasicDBObject unwind = new BasicDBObject();
unwind.put("$unwind", "$requests");
aggregationInput.add(unwind);

BasicDBObject groupVal = new BasicDBObject();
groupVal.put("_id", "$_id");
groupVal.put("count", new BasicDBObject().put("$sum", 1));

BasicDBObject group = new BasicDBObject();
group.put("$group", groupVal);
aggregationInput.add(group);

AggregationOutput output = followRequestsCol.aggregate(aggregationInput);
for (DBObject result : output.results()) {
    System.out.println(result);
}

Or, slightly more readable, use the fluent BasicDBObjectBuilder:

final DBObject match = BasicDBObjectBuilder.start()
                                           .push("$match")
                                               .add("_id", new ObjectId())
                                           .get();
aggregationInput.add(match);

And it should work fine.

Excellent, thanks a ton, it works. Just had to do the same thing with group val for count also.

Valijon · Accepted Answer · 2016-01-10 18:07:21Z

1

Each {} must be new DBObject. Use also .append(key,value) method to make more elegant.

Try this:

List<DBObject> pipeline = new ArrayList<DBObject>(Arrays.asList(
    new BasicDBObject("$match", new BasicDBObject("_id", 
        new ObjectId("551e78c6de5150da91c78ab9"))), 
    new BasicDBObject("$unwind", "$requests"),
    new BasicDBObject("$group", 
        new BasicDBObject("_id","$_id").append("count", new BasicDBObject("$sum", 1)))));

AggregationOutput output = followRequestsCol.aggregate(pipeline);
for (DBObject result : output.results()) {
    System.out.println(result);
}

answered Jan 10, 2016 at 18:07

Valijon

13.2k4 gold badges40 silver badges74 bronze badges

Comments

PavanMysore · Accepted Answer · 2016-01-10 19:16:12Z

This is the final working version, based on the above suggestions // Use mongodb aggregation framework to determine the count of followers Integer returnCount = 0; List aggregationInput = new ArrayList();

    BasicDBObject match = new BasicDBObject();
    BasicDBObject matchQuery = new BasicDBObject();
    matchQuery.put("_id", new ObjectId(clientId));
    match.put("$match", matchQuery);
    aggregationInput.add(match);

    BasicDBObject unwind = new BasicDBObject();
    unwind.put("$unwind", "$requests");
    aggregationInput.add(unwind);

    BasicDBObject groupVal = new BasicDBObject();
    groupVal.put("_id", null);
    BasicDBObject sum = new BasicDBObject();
    sum.put("$sum", 1);
    groupVal.put("count", sum);
    BasicDBObject group = new BasicDBObject();
    group.put("$group", groupVal);
    aggregationInput.add(group);

    AggregationOutput output = followRequestsCol.aggregate(aggregationInput);
    for (DBObject result : output.results()) {
        returnCount = (Integer) result.get("count");
        break;
    }
    return returnCount;

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