Mongodb select all fields group by one field and sort by another field

Question

We have collection \'message\' with following fields

_id |   messageId |  chainId | createOn

1   |       1     |    A     | 155
2   |       2     |    A     | 1         


        

Answer 1

here is the solution using MongoDB Java Driver

    final MongoClient mongoClient = new MongoClient();
    final DB db = mongoClient.getDB("mstreettest");
    final DBCollection collection = db.getCollection("message");

    final BasicDBObject groupFields = new BasicDBObject("_id", "$chainId");
    groupFields.put("docId", new BasicDBObject("$first", "$_id"));
    groupFields.put("messageId", new BasicDBObject("$first", "$messageId"));
    groupFields.put("createOn", new BasicDBObject("$first", "$createdOn"));

    final DBObject group = new BasicDBObject("$group", groupFields);

    final DBObject sortFields = new BasicDBObject("createOn", -1);
    final DBObject sort = new BasicDBObject("$sort", sortFields);

    final DBObject projectFields = new BasicDBObject("_id", 0);
    projectFields.put("_id", "$docId");
    projectFields.put("messageId", "$messageId");
    projectFields.put("chainId", "$_id");
    projectFields.put("createOn", "$createOn");
    final DBObject project = new BasicDBObject("$project", projectFields);

    final AggregationOutput aggregate = collection.aggregate(group, sort, project);

and the result will be:

{ "_id" : 5 , "messageId" : 5 , "createOn" : { "$date" : "2014-04-23T04:45:45.173Z"} , "chainId" : "C"}
{ "_id" : 4 , "messageId" : 4 , "createOn" : { "$date" : "2014-04-23T04:12:25.173Z"} , "chainId" : "B"}
{ "_id" : 1 , "messageId" : 1 , "createOn" : { "$date" : "2014-04-22T08:29:05.173Z"} , "chainId" : "A"}

I tried it with SpringData Mongo and it didn't work when I group it by chainId(java.lang.NumberFormatException: For input string: "C") was the exception