Java insert value to Array in MongoDB

Question

im new to MongoDB and completely confused by the queries. I simply need to update a document in a mongodb database by adding a string value (example: Temperature) to a list of strings. From research I know that I have to use the $push method for that. I think the code has to look somehow like this:

BasicDBObject newDocument = new BasicDBObject().append("$set", 
new BasicDBObject().append("Subscribed Topics", topic));

collection.update(new BasicDBObject().append("Sensor Type", sensorType), newDocument);
new BasicDBObject("$push",
new BasicDBObject("Subscribed Topics", topic));

The field with the array is called "Subscribed Topics", "topic" is a String (Temperature). Then I want to update the document in the collection with the corresponding "Sensor Type". However, I do not really know how to call the $push part correctly. I hope someone can help me sort this part of the code.

Best regards.

Update, I tried to implemented as suggested in the duplicate question but still got error. Very unsure if thats the right way anyway.

 DBObject listItem = new BasicDBObject("Subscribed Topics", "Light");
             DBObject updateQuery = new BasicDBObject("$push", listItem);
             collection.update(query, updateQuery);`

I create a new Object with the value Light in for Key Subscribed Topics (the array). Why do I push it to a new Object then?

Answer 1

My goodness! This question got me descending into the long forgotten world of Java again - after all these years... ;) Anyhoo, here's a complete working example that might give you a clue of what's going on. You can run the code several times and see how the number of elements in the "Subscribed Topics" array increases.

I used the following driver: https://oss.sonatype.org/content/repositories/releases/org/mongodb/mongo-java-driver/3.3.0/mongo-java-driver-3.3.0.jar

import com.mongodb.MongoClient;
import com.mongodb.client.MongoDatabase;
import com.mongodb.client.MongoCollection;

import org.bson.Document;
import org.bson.conversions.Bson;

import static com.mongodb.client.model.Filters.*;
import static com.mongodb.client.model.Updates.*;

public class MongoDbPush {
  public static void main(String[] args)
  {
    MongoClient mongoClient = new MongoClient();
    MongoDatabase database = mongoClient.getDatabase("pushExampleDb");
    MongoCollection<Document> collection = database.getCollection("pushExampleCollection");

    String sensorType = "Temperature";

    // try to load existing document from MongoDB
    Document document = collection.find(eq("Sensor Type", sensorType)).first();
    if(document == null)
    {
      // no test document, let's create one!
      document = new Document("Sensor Type", sensorType);

      // insert it into MongoDB
      collection.insertOne(document);

      // read it back from MongoDB
      document = collection.find(eq("Sensor Type", sensorType)).first();
    }

    // see what it looks like in JSON (on the first run you will notice that it has got an "_id" but no "Subscribed Topics" array yet)
    System.out.println(document.toJson());

    // update the document by adding an entry to the "Subscribed Topics" array
    Bson filter = eq("Sensor Type", sensorType);
    Bson change = push("Subscribed Topics", "Some Topic");
    collection.updateOne(filter, change);

    // read one more time from MongoDB
    document = collection.find(eq("Sensor Type", sensorType)).first();

    // see what the document looks like in JSON (on the first run you will notice that the "Subscribed Topics" array has been created and has got one element in it)
    System.out.println(document.toJson());

    mongoClient.close();
  }
}