Getting the size of arraylist from a MongoDB table

Question

I am trying to get the count of followers from user table which is basically a array list using java programming.

I am using below query to get the count using command line interface.

db.userinfo.aggregate([{ $project : {followersCount : {$size: "$followers"}}}])

But I am not able to create the same query in java as I am new. Below is the code I wrote in java and I am getting com.mongodb.MongoCommandException: Command failed with error 17124: 'The argument to $size must be an array, but was of type: int' Error.

AggregateIterable<Document> elements = collectionUserInfo.aggregate(
                Arrays.asList(new BasicDBObject("$project", new       BasicDBObject("followers",
                        new BasicDBObject("$size", new BasicDBObject("followers",1).put("followers",new BasicDBObject("$ifNull", "[]")))))));

Can Anyone please help me with this

I tried this and it worked fine.

Shravya
– Shravya

2017-01-10 01:26:35 +00:00
Commented Jan 10, 2017 at 1:26 — Shravya
– Shravya, Commented Jan 10, 2017 at 1:26

s7vr · Accepted Answer · 2017-01-11 22:38:51Z

5

You can try something like with Java 8 and Mongo Driver 3.x version. You should try not to use old type ( BasicDbObject) and where possible use api method.

List<Integer> followersCount = collectionUserInfo.aggregate(
            Arrays.asList(Aggregates.project(Projections.computed(
                    "followersCount",
                    Projections.computed("$size", "$followers"))
                    )
            ))
   .map(follower -> follower.getInteger("followersCount"))
   .into(new ArrayList<>());

edited Jan 11, 2017 at 22:38

answered Jan 10, 2017 at 1:57

s7vr

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1 Comment

Shravya Over a year ago

the above code worked for me only after adding $ in front of followers at new Docment("$size", "$followers").

Shravya · Accepted Answer · 2017-01-10 01:27:44Z

1

I tried this and it worked fine.

AggregateIterable<Document> elements = collectionUserInfo.aggregate(
                Arrays.asList(new BasicDBObject("$project", new BasicDBObject("followers",
                        new BasicDBObject("$size", "$followers")))));

Thanks

answered Jan 10, 2017 at 1:27

Shravya

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