Fetch _id of a Document in MongoDB 3.3.0 above

Question

Trying to refactor a lot of code from one of our services I ended up using a piece of code as :

Document documentObject;
String docMongoId = ((DBObject) documentObject).removeField("_id").toString();

which though not on compiling but during execution has resulted into

java.lang.ClassCastException: com.pack.model.Document cannot be cast to com.mongodb.DBObject

wherein Document is a type defined by us. We now have a piece of code as follows :

MongoCollection<Document> dbCollection = mongoClient.getDatabase("dbName")
                .getCollection("collectionName", Document.class);

Seeking help, already went through few links -

Get ID of last inserted document in a mongoDB w/ Java driver

MongoError: cannot change _id of a document

How to query documents using "_id" field in Java mongodb driver?

which mostly suggest using BasicDBObject and we don't want to be using the older context anymore. Is there a way I can access the _id of the Document now(with client update) in java?

Please do ask for any additional or missing information required here.

what exactly are you trying to refactor ? Can you add the old code too ? please add an example. — s7vr, Nov 28 '16 at 14:04
@Veeram the intent is to keep the _id of a document and preserve it while either updating an existing doc or creating a new doc if there is none matching the search. The _id is also shared in another doc `Client` which basically marks the presence of latest doc. — Naman, Nov 29 '16 at 07:16
Thank you for the explanation. Let me know if you need me to improve my answer. — s7vr, Nov 29 '16 at 16:42

score 2 · Accepted Answer · answered Nov 28 '16 at 14:15

The idea is to convert from bson type Document to your model Document. Added sample class demonstrating the use.

import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoCursor;
import com.mongodb.client.MongoDatabase;
import org.bson.types.ObjectId;

public class Mongo {

    public static void main(String[] args) {

        MongoClient mongoClient = new MongoClient();

        MongoDatabase db = mongoClient.getDatabase("dbName");

        MongoCollection <org.bson.Document> dbCollection = db.getCollection("collectionName", org.bson.Document.class);

        MongoCursor <org.bson.Document> iterator = dbCollection.find().iterator();

        while (iterator.hasNext()) {
            org.bson.Document inDoc = iterator.next();
            Document outDoc = new Document();
            outDoc.setId(inDoc.getObjectId("_id"));
            outDoc.setName(inDoc.getString("name"));
            System.out.print(outDoc.toString());
        }

    }


    public static class Document {

        private ObjectId id;
        private String name;

        public ObjectId getId() {
            return id;
        }

        public void setId(ObjectId id) {
            this.id = id;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        @Override
        public String toString() {
            return "Document{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
        }

    }
}

Fetch _id of a Document in MongoDB 3.3.0 above

1 Answers1