How to retrieve / find all elements of a nested array in MongoDB Java

Question

I am trying to find / load all elements in the "nodes" array into a Java List or Hashmap.

I'm dealing with a specific JSON format that I cannot modify. The Mongo DB collection contains only one document, and that document is shown below. I am trying to query all elements of the "nodes" array but can't manage to do so.

MongoCollection<Document> collection = mongoDB.getCollection(collectionName); 
BasicDBObject query = new BasicDBObject();
query.put("nodes", "");
List<Document> test2 = collection.find(query).into(new ArrayList<Document>());

Test2 returns NULL at the moment. I know I'm wrong but can't figure out how to do it. And here is the JSON

{
  "_id": "12123434",
  "nodes": [
    {
      "id": "1",
      "name": "bla",
      "attributes": [
        "string1",
        "string2"
      ]
    },
    {
      "id": "2",
      "name": "blabla",
      "attributes": [
        "string1",
        "string2"
      ]
    }
  ],
  "groups": []
}

s7vr · Accepted Answer · 2017-01-29 00:52:46Z

2

You just need to project nodes and map.

import static com.mongodb.client.model.Projections.*;

List<Document> nodes = (List<Document>) collection.find().projection(fields(include("nodes"), excludeId())).map(document -> document.get("nodes")).first();

edited Jan 29, 2017 at 0:52

answered Jan 29, 2017 at 0:43

s7vr

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2 Comments

Storm Over a year ago

I cant get the methods include() and excludeId() to be recognized. I tried importing everything but it still won't work

s7vr Over a year ago

Added the static import to the answer.

Ali · Accepted Answer · 2020-04-16 17:01:04Z

0

Thank you Sagar, your code snippet helped me a lot. Just in case if someone wants to target the filtering on a field (ex - id) within the nodes document then the following can be used.

List<Document> nodes =  (List<Document>) collection.find().projection(elemMatch("nodes",new Document("id",2))).map(document -> document.get("nodes")).first();

answered Apr 16, 2020 at 17:01

Ali

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