MongoDB java Driver query on nested object using objectID is not working

Question

I have following mongodb document structure

{
    "_id": ObjectId("571530da41995a703faaf55b"),
    "project_name": "abc",
    "status": "new",
    "cluster": "",
    "sku": {
        "_id": "57152f7941995a703faaf559",
        "name": "temp",
        "distribution": "apache",
        "creation_date": "2016-04-18T19:02:46.595Z",
        "user_id": "570784b1682d25f45b64ed51",
        "__v": 0,
        "services": []
    }
}

I want to find project info given a sku._id. I am using following piece of code for this purpose.

String  getProjectIDforSku(String skuID ) {
        ObjectId id = new ObjectId(skuID);
        BasicDBObject query = new BasicDBObject("sku", new BasicDBObject("_id", skuID));
        DBObject obj = getCollection("deployment", "projects").findOne(query);
        if (obj != null) {
            var jsonString = Json.parse(obj.toString());
            return jsonString;
        } else
            return null;
    }

When I call this function, I receive null instead of valid json string. Can anyone guide me where i am wrong? Thanks

Answer 1

Change BasicDBObject query = new BasicDBObject("sku", new BasicDBObject("_id", skuID)); to BasicDBObject query = new BasicDBObject("sku._id",skuID);

The reason being that in the first one you are comparing sku to an object with only one field _id (which is not the case, the object has other fields), while the second one only matches the _id field within the sku object.