Run an aggregation using Java Driver - MongoDB

Question

Using MongoDB shell I use:

db.bios.aggregate(
[
{$match:{"contribs.0.name":{"$exists":1}}}, 
{$project: {contribs:{$arrayElemAt:["$contribs",0]}}}
]
)

How can I make the same query using Java driver (2.14.1)? I try with:

At first I create a DBObject for $match stage:

DBObject match = new BasicDBObject("$match",new BasicDBObject("contribs.0.name",
                 new BasicDBObject("$exists",1)));

Then I create a BasicDBList:

BasicDBObject obj = new BasicDBObject("$contribs",0);

BasicDBList arrayElemAt = new BasicDBList();
arrayElemAt.add(obj);

And this is the $project stage:

DBObject project1 = new BasicDBObject("$project", new BasicDBObject("contribs",
                    new BasicDBObject("$arrayElemAt",arrayElemAt)));

Finally I create the aggregation pipeline:

List<DBObject> list = new ArrayList<>();
list.add(match);
list.add(project1);

AggregationOutput output = this.coll.aggregate(list);

$Match stage works, but $project does not.

I get an error: "errmsg" : "invalid operator '$contribs'" , "code" : 15999

Blakes Seven · Accepted Answer · 2016-03-14 21:08:08Z

1

You created a DBObject when you were meant to create a List. Also DBList has been deprecated for a while now. Get used to using standard list notations:

    List<DBObject> pipeline = Arrays.<DBObject>asList(
        new BasicDBObject(
            "$match",
            new BasicDBObject(
                "contribs.0", new BasicDBObject("$exists",true)
            )
        ),
        new BasicDBObject(
            "$project",
            new BasicDBObject(
                "contribs", new BasicDBObject("$arrayElemAt", Arrays.asList("$contribs",0))
            )
        )
    );

    AggregationOutput output = this.coll.aggrgate(pipeline);

Also note that in modern drivers you really should be using Document in place of all DBObject types.

answered Mar 14, 2016 at 21:08

Blakes Seven

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1 Comment

harry-potter Over a year ago

Thank you very much. I was stuck for two hours. I know in Java Driver 3.0 there are new features, but I started my project with 2.14.1. Then I'm going to pass to Java Driver 3.0+.

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