how to sort array inside collection record in mong

Question

MongoDB noob here...

Ok, I have a collection of students, each with a record that looks like the following.... I want to sort the 'type' : 'homework' scores in descending order.

what does that incantation look like on the mongo shell?

> db.students.find({'_id': 1}).pretty()
{
        "_id" : 1,
        "name" : "Aurelia Menendez",
        "scores" : [
                {
                        "type" : "exam",
                        "score" : 60.06045071030959
                },
                {
                        "type" : "quiz",
                        "score" : 52.79790691903873
                },
                {
                        "type" : "homework",
                        "score" : 71.76133439165544
                },
                {
                        "type" : "homework",
                        "score" : 34.85718117893772
                }
        ]
}

I'm trying this incantation....

 doc = db.students.find()

 for (_id,score) in doc.scores:
     print _id,score

but it's not working.

Answer 1

Here is the java code which can be used to find out the lowest score in the array and remove it.

public class sortArrayInsideDocument{
public static void main(String[] args) throws UnknownHostException {
    MongoClient client = new MongoClient();
    DB db = client.getDB("school");
    DBCollection lines = db.getCollection("students");
    DBCursor cursor = lines.find();
    try {
        while (cursor.hasNext()) {
            DBObject cur = cursor.next();
            BasicDBList dbObjectList = (BasicDBList) cur.get("scores");
            Double lowestScore = new Double(0);
            BasicDBObject dbObject = null;
            for (Object doc : dbObjectList) {
                BasicDBObject basicDBObject = (BasicDBObject) doc;
                if (basicDBObject.get("type").equals("homework")) {
                    Double latestScore = (Double) basicDBObject
                            .get("score");
                    if (lowestScore.compareTo(Double.valueOf(0)) == 0) {
                        lowestScore = latestScore;
                        dbObject = basicDBObject;

                    } else if (lowestScore.compareTo(latestScore) > 0) {
                        lowestScore = latestScore;
                        dbObject = basicDBObject;
                    }
                }
            }
            // remove the lowest score here.
            System.out.println("object to be removed : " + dbObject + ":"
                    + dbObjectList.remove(dbObject));
            // update the collection
            lines.update(new BasicDBObject("_id", cur.get("_id")), cur,
                    true, false);
        }
    } finally {
        cursor.close();
    }
}
}

Answer 2

Certainly it's late, but I just want to contribute my own solution on Mongo Shell:

var students = db.getCollection('students').find({});
for(i = 0 ; i < students.length(); i++) {
    var scores = students[i].scores;
    var tmp = [];
    var min = -1 ;
    var valueTmp = {};
    for(j = 0 ; j < scores.length; j++) {        
        if(scores[j].type != 'homework') {
            tmp.push(scores[j]);
        } else {
            if (min == -1) {
                min = scores[j].score;
                valueTmp = scores[j];
            } else {
                if (min > scores[j].score) {
                    min = scores[j].score;
                    tmp.push(valueTmp);
                    valueTmp = scores[j];
                } else {
                    tmp.push(scores[j]);
                }
            }
        }
    }
    db.students.updateOne({_id:students[i]._id},
                            {$set:{scores:tmp}});
}

Answer 3

it should be something like this:

db.students.find().sort(scores: ({"score":-1}));

Answer 4

That's how we could solve this with JS and mongo console:

db.students.find({"scores.type": "homework"}).forEach(
  function(s){
    var sortedScores = s.scores.sort(
      function(a, b){
        return a.score<b.score && a.type=="homework";
      }
    );
    var lowestHomeworkScore = sortedScores[sortedScores.length-1].score;
    db.students.update({_id: s._id},{$pull: {scores: {score: lowestHomeworkScore}}}, {multi: true});
  })

Answer 5

the answer of @Stennie is fine, maybe a $group operator would be useful to keep the original document, without exploding it in many documents (one by score).

I just add another solution when using javascript for your application.

if you query only one document, it's sometimes easier to sort the embedded array by JS, instead of doing an aggregate. When your document has a lot of fields, it's even better than using $push operator, otherwise you've to push all the fields one by one, or use $$ROOT operator (am I wrong ?)

My example code uses Mongoose.js : Suppose you have initialized you Students model.

// Sorting
function compare(a, b) {
  return a.score - b.score;
}

Students.findById('1', function(err, foundDocument){
  foundDocument.scores = foundDocument.scores.sort(compare);

  // do what you want here...
  // foundModel keeps all its fields
});

Answer 6

Since this question can be managed in different ways i want to say that another solution is "insert and sort", in this way you will get the Ordered array at the moment you will made a Find().

Consider this data:

{
   "_id" : 5,
   "quizzes" : [
      { "wk": 1, "score" : 10 },
      { "wk": 2, "score" : 8 },
      { "wk": 3, "score" : 5 },
      { "wk": 4, "score" : 6 }
   ]
}

Here we will update the Document, make the Sort.

db.students.update(
   { _id: 5 },
   {
     $push: {
       quizzes: {
          $each: [ { wk: 5, score: 8 }, { wk: 6, score: 7 }, { wk: 7, score: 6 } ],
          $sort: { score: -1 },
          $slice: 3 // keep the first 3 values
       }
     }
   }
)

Result is:

{
  "_id" : 5,
  "quizzes" : [
     { "wk" : 1, "score" : 10 },
     { "wk" : 2, "score" : 8 },
     { "wk" : 5, "score" : 8 }
  ]
}

Documentation: https://docs.mongodb.com/manual/reference/operator/update/sort/#up._S_sort