Java -- MongoDB collection.find() by _id

Question

I am trying to get an element from a collection by using his unique _id, but I can not find how.

This is my code

MongoClient mongoClient = new MongoClient("localhost", 27017);
MongoDatabase database = mongoClient.getDatabase("DB");
MongoCollection<Document> collection =   database.getCollection("COLL");

If I query my db with

BasicDBObject query=new BasicDBObject("info.i0","0");
Document myDoc = collection.find(query).first();
System.out.println(myDoc.toJson());

I get as output

{ "_id" : { "$oid" : "560ea3f205240f065a3e9d19" }, "name" : "MongoDB", "type" : "database", "count" : 1, "info" : { "i0" : "0", "i1" : "1", "i2" : "2", "i3" : "3", "i4" : "4", "i5" : "5", "i6" : "6", "i7" : "7", "i8" : "8", "i9" : "9" } }

But If I try

BasicDBObject query=new BasicDBObject("_id.$oid","560ea3f205240f065a3e9d19");
Document myDoc = collection.find(query).first();
System.out.println(myDoc.toJson());

I get a Null pointer exception, as myDoc is null.

What am I doing wrong?

Answer 1

$oid is there only to preserve BSON representation.

It only has meaning to MongoDB internal JSON parsers.

You only have to use _id in your query:

BasicDBObject query=new BasicDBObject("_id",new ObjectId("560ea3f205240f065a3e9d19"));

Also, note that the _id field is of type ObjectId and not String.

You will have to wrap it in an ObjectId constructor.