Sorting DBList using Collections.sort

Question

I have a DBList that contains DBObjects from a cursor = coll.find() query. I have a field named timestamp inside each DBObject and I want to sort the DBList by . 对DBList进行排序。 I know I can use Collections.sort() here but being new to mongoDB's java API, I can't seem to be able to sort based on a specific field. I would really appreciate the help.

The unsorted list was formed using this :

      DBCursor cursor = coll.find(whereQuery);                  

      while(cursor.hasNext()) {
          DBObject o = cursor.next();
          System.out.println(o.toString());
          unsortedList.add(o);

      }

After this, I want to sort unsortedList in the descending order of timestamp . I'm quite unclear on how to do this and thanks in advance for the help.

Also solutions, such as coll.find().sort(timestamp,-1) can't be used because there is an outer for loop which keeps changing the whereQuery . The list must be sorted only after the query is done.

Answer 1

The problem is that how do you change your query. Could you show me the code that you change your query in the for loop, I think i can provide a better query for you. MongoDB can support very complicated queries.

Strill, I think a better way is letting mongoDB sort it for you. You can write like:

coll.find(whereQueryModified).sort(new BasicDBObject("timestamp",-1));

If you cannot use the sort method, you might need to use a newer version of mongoDB driver.

Besides, "timestamp" is to long for a key of an object. It's very space exhausting for mongoDB because mongoDB store every key and value in disk.

Answer 2

You can change the query as following strategy:

List<DBObject> pipeline=new ArrayList<DBObject>();
DBObject match = new BasicDBObject("$match", new BasicDBObject("date", sdf.format(new Date())).append("country", country));
DBObject unwind = new BasicDBObject("$unwind", "$details");
DBObject sort = new BasicDBObject("$sort", new BasicDBObject("details.datetime", -1));
DBObject limit = new BasicDBObject("$limit", 1);

pipeline.add(match);
pipeline.add(unwind);
pipeline.add(sort);
pipeline.add(limit);

AggregationOutput outputoutput = collection.aggregate(pipeline);

Answer 3

如果DBList实现List ：

Collections.sort(unsortedList, Comparator.comparing(DBObject::getTimestamp).reversed());