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Cannot get _id of MongoDB object with java driver

I'm inserting objects into MongoDB without specifying the _ids, because I want it to create them automatically. The problem is that at a certain point of the program I need to retrieve the _ids, but I can't get them. The code I use is the following:
List<DBObject> objs=collection.find(filter).toArray();
BDObject obj=objs.get(0);
String id=obj.get("_id");
//now id is something like 2d938830-2732-44fd-84b0-aa56b95c5df0
Eventually the id variable contains a GUID, but it's different from the one I see in RoboMongo, so it's wrong. What I see in RoboMongo is something like:
"_id": LUUID("cada0d4f-a72d-47ad-8ea8-239c3e5795dd")

Mongo DB / No duplicates

I have have a mongo collection that keeps state records for devices. Thus, there could be multiple records per device. What I would like to do is create a query through the mongoTemplate that gets the latest record for each device.
Here's the constraints:
Pass in a Set<'String'> name_ids, regular field within mongo collection not the _id or found within the _id
get only the latest record for each device with matching name_id
return List<'DeviceStateData'> (No duplicates should be found with the same name_id)
example of collection object:
{
_id: "241324123412",
name_id: "flyingMan",
powerState:"ON",
timeStamp: ISODate('')
}
Thanks

You should look on Distinct function.
Here you can find details with Spring.

kairosdb and elasticsearch integration

I'm using Kairosdb as my primary db. Now I want to integrate the Elasticsearch functionalities to my data inside Kairosdb. As stated inside the docs I have to duplicate all entries of my primary db inside Elasticsearch database.
Update
What I mean is that, if I want to index something inside elasticsearch, I have to do, for example:
Retrieve data of Kairosdb, a example json {"name": "hi","value": "6","tags"}
and then put it inside Elasticsearch:
curl -XPUT 'http://localhost:9200/firstIndex/test/1' -d '{"name": "hi","value": "6","tags"}'
If I want to search I have to do this:
curl 'http://localhost:9200/_search?q=name:hi&pretty=true'
I'm wondering if it is possible to not duplicate my data inside Elasticsearch, in a way which I can achieve this:
get data from KairosDB
index them using Elasticsearch without duplicate the data.
How can I go about that?

It sounds like you're hoping to use Elasticsearch as a secondary (and external) fulltext index for your primary datastore (KairosDB).
Since KairosDB is remaining your primary datastore, each record you load into Elasticsearch needs two pieces of information (at minimum):
The primary key field(s) for locating the corresponding KairosDB record(s). In the mapping, make sure to set "store": true, "index": "not_analyzed"
Any fields which you wish to be searchable (in your example, only name is searched) "store": false, "index": "analyzed"
If you want to reduce your index size further, consider disabling the _source field
Then your search workflow becomes a two-step process:
Query Elasticsearch for name:hi and retrieve the KairosDB primary key field(s) for each of the matching record(s).
Query/return KairosDB time-series data using key fields returned from Elasticsearch.
But to be clear. You don't need an exact duplicate of each KairosDB record loaded into Elasticsearch. Just the searchable fields, along with a means to locate the original record in KairosDB.

MongoDB: insert documents with specific id instead of auto generated ObjectID

I need to insert documents on MongoDB (with specific id instead of auto generated ObjectID) using java..
To insert one document or update if exist, I tried use findOne to search for the id, if it doesn't exist then insert the id and then findAndModify. It works but I don't feel that it's efficient way, it's time consuming. Is there a better way to achieve that?
To insert multiple documents at once,I'm following this solution. but I don't know how I can insert my custom Id instead of objectID?
Any help will be appreciated

For your first problem MongoDB has upsert so
db.collection.update(
{query for id},
{document},
{upsert: true}
)
or in the Java driver
yourCollection.update(searchObject, modifiedObject, true, false);
If you want to set a custom ID you just set the _id key manually i.e.
yourBasicDBObject.put("_id",yourCustomId)
you just have to ensure it is unique for each document.
You will also need to set the _id in your modifiedObject otherwise a new one will be generated.
As for the bulk operations, just setting a custom ID for each document by giving the _id key should also work.

Try this #ebt_dev:
db.collection("collectionname").insertOne(data, { forceServerObjectId: false })

Transform Cassandra query result to POJO with Astyanax

I am working in a Spring web application using Cassandra with Astyanax client. I want to transform result data retrieved from Cassandra queries to a POJO, but I do not know which library or Astyanax API support this.
For example, I have User column family (CF) with some basic properties (username, password, email) and other related additional information can be added to this CF. Then I fetch one User row from that CF by using OperationResult> to hold the data returned, like this:
OperationResult<ColumnList<String>> columns = getKeyspace().prepareQuery(getColumnFamily()).getRow(rowKey).execute();
What I want to do next is populating "columns" to my User object. Here, I have 2 problems and could you please help me solve this:
1/ What is the best structure of User class to hold the corresponding data retrieved from User CF? My suggestion is:
public class User {
String userName, password, email; // Basic properties
Map<String, Object> additionalInfo;
}
2/ How can I transform the Cassandra data to this POJO by using a generic method (so that it can be applied to every single CF which has mapped POJO)?
I am so sorry if there are some stupid dummy things in my questions, because I have just approached NoSQL concepts and Cassandra as well as Astyanax for 2 weeks.
Thank you so much for your help.

You can try Achilles : https://github.com/doanduyhai/achilles, an JPA compliant Entity Manager for Cassandra
Right now there is a complete implementation using Thrift API via Hector.
The CQL3 implementation using Datastax Java Driver is in progress. A beta version will be available in few months (July-August 2013)
CQL3 is great but it's still too low level because you need to extract the data yourself from the ResultSet. It's like coming back to the time when only JDBC Template was available.
Achilles is there to fill the gap.

I would suggest you to use some library like Playorm using which you can easily perform CRUD operations on your entities. See this for an example that how you can create a User object and then you can get the POJO easily by
User user1 = mgr.find(User.class, email);
Assuming that email is your NoSqlId(Primary key or row key in Cassandra).

I use com.netflix.astyanax.mapping.Mapping and com.netflix.astyanax.mapping.MappingCache for exactly this purpose.