How can I fetch the size of array in mongodb using springboot?

Question

I'm new to springboot and mongodb as well. I have the following json document in mongodb

Note: *Name of database is Friends and name of collection is Friend. It has around 118k documents.

One sample document:

[{
"_id":"abhj",
"id":"abhj",
"Main_array":[
    {
      "number":12345,
      "pincode":247800,
      "address": [
         "vasant"
         "vihar"
         "kota"
        ]
      }
     ],
}]

There is Main_array inside which there is an object inside which we have address which is an array.

I want to fetch the size of this address array.

I tried this but it didn't work.

Note: I have to use MongoClient.

MongoDatabase database = mongoClient.getDatabase("Friends");
        MongoCollection<Document> collection = database.getCollection("Friend");

        BasicDBObject filter = new BasicDBObject("Main_Array.0.address", new BasicDBObject("$exists", "true"));
        collection.find(filter).forEach((Consumer<Document>) doc -> { 
                                Object obj = doc.get("Main_array.address")
}

But I got null value in obj.

Its better if you post your sample data in json format and post your expected output — varman
– varman, Commented May 19, 2022 at 9:06
Done bro. Just want to fetch the size of the "address" array — Rishabh
– Rishabh, Commented May 19, 2022 at 9:20

varman · Accepted Answer · 2022-05-19 09:56:23Z

1

You can use following aggregation to find out the size.

here is the code

db.collection.aggregate([
  {
    $addFields: {
      Main_array: {
        "$map": {
          "input": "$Main_array",
          "in": {
            number: "$$this.number",
            pincode: "$$this.pincode",
            addressSize: {
              $size: "$$this.address"
            }
          }
        }
      }
    }
  }
])

Working Mongo playground

There is a TRICK to convert aggregations... The java code might be

@Autowired
private MongoTemplate mongoTemplate;

public List<YOUR_CONVERTER_CLASS> test() {

    Aggregation aggregation = Aggregation.newAggregation(
        l-> new Document("$addFields",
                new Document("Main_array",
                    new Document("$map",
                        new Document("input","$Main_array")
                        .append("in",
                            new Document("number","$$this.number")
                            .append("pincode","$$this.pincode")
                            .append("addressSize",
                                new Document("$size","$$this.address")
                            )
                        )
                    )
                )
        )
    ).withOptions(AggregationOptions.builder().allowDiskUse(Boolean.TRUE).build());

    return mongoTemplate.aggregate(aggregation, mongoTemplate.getCollectionName(YOUR_COLLECTION_CLASS.class), YOUR_CONVERTER_CLASS.class).getMappedResults();

}

edited May 19, 2022 at 9:56

answered May 19, 2022 at 9:51

varman

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5 Comments

Rishabh Over a year ago

This really seems to be promising. Thank you soo much for answering. But just want to tell the colletion is actually a private collection for which I haven't created a class and even I was asked not create class for the collection. Is there any chance we can use MongoCursor or any other method by which we do not have to create a class of the collection

Rishabh Over a year ago

and I guess we are creating Document and appending ...there are 118k documents. I guess it will be soo much work.

Rishabh Over a year ago

Hello @varman .I will be highly grateful to you if you can suggest something. Those array with size less than 3 for those I need to perform a java function as well. Do you know anything which I can use ?

varman Over a year ago

You may edit this question or may open another question and show me what you need, (sample data and expected output)

Rishabh Over a year ago

Okay sure means a lot to me. I have already created the question -- stackoverflow.com/questions/72364406/…

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