Saving with Java springdata a mongoDB document with capped array ($slice and $sort)

Question

I'm developing a log register using mongoDB and Java SpringData.

Here MongoDb capped sub-collection talks about mongoDB structure, but I would do with Java. The most important thing it's that I have a document with one or more fields and a capped array.

Is there some method or way in Java to do this?

My object it's like:

user = {
   name: String,
   latest_messages: [String] (capped to a 100 elements)
}

in Java:

public class MessageLog {
    private ObjectId id;
    private String name;
    private List<Message> messages;
}

Where:

public class Message{
    private String text;
    private String level;
    private Date date;
}

EDIT :

I'm using Java with Spring source (mongodb driver v2.10.1: http://central.maven.org/maven2/org/mongodb/mongo-java-driver/2.10.1/ )

Answer 1

I finally get the solution using this code:

// Define the search query
BasicDBObject searchQuery = new BasicDBObject().append("idU", idUser);

// To create the json query to modify
BasicDBObject logDocument = new BasicDBObject();

// Create the object and add it to a list (because the $each require a list)
List<DBObject> list = new ArrayList<DBObject>();
DBObject object = new BasicDBObject().append("text", logMessage.getText());
object.append("level", logMessage.getLevel())
object.append("date", logMessage.getDate());
list.add(object);

// Create the $sort and $slice query at the same time
logDocument.append(
    "$push",
    new BasicDBObject().append("logs", new BasicDBObject().append("$each", list).append("$sort", new BasicDBObject().append("date", 1))
            .append("$slice", -10)));

String json = "{findAndModify:\"collectionLog\", query:" + searchQuery.toString() + ", update: " + logDocument.toString() + ", upsert:true}";

try {
     getMongoTemplate().executeCommand(json);
} catch (Exception e) {
     System.out.println(e);
}

I hope it will be useful to someone else!!!

Answer 2

As you have noticed, MongoDB can't provide this functionality, so it has to be solved on the application layer.

Java by default doesn't support anything which works like a capped collection. But the collection library of the Apache Commons project has the class CircularFiFoBuffer which seems to be what you need. You create it with a max size. When you add a new element and it already is at the max size, the oldest element is discarded. Iteration order is from the oldest element to the newest.

A workaround without external libraries could be done with a LinkedList . Add new elements using the addFirst() method, check the size, and when it's larger than the desired maximum size, call removeLast().