// Populate :

var testcol = db.getCollection('Test1');
for (var i = 0; i < 100000; i++) {
    var date = new Date((1496211798264 + i));
    var item = {
        subject: "Test " + i,
        events: [],
        update: date

    };

    for (var j = 0; j < 15; j++) {
        item.events.push({
            event: "XXXX" + j,
            date: new Date(date-j),
        });
    }

    testcol.insert(item);
}


// Indexes

index :
{
    "events.0.date" : 1
}

index :

{
    "update" : 1
}


// Sort "events.0.date" with "_id"

testcol.aggregate([
			{
                            $sort:{
                                "events.0.date": -1,
                                '_id':1
                            }
                        },
                        {
                               $limit:4
                         },
			{
                            $project:{
                            "subject":true,
                            "update": {
                                "$arrayElemAt": ["$events.date", 0]
                                }
                            }
                        }
                     ])

//wrong result
/* 1 */
{
    "_id" : ObjectId("592e7d182fc987735f3dda4d"),
    "subject" : "Test 0",
    "update" : ISODate("2017-05-31T10:23:18.264+04:00")
}

/* 2 */
{
    "_id" : ObjectId("592e7d182fc987735f3dda4e"),
    "subject" : "Test 1",
    "update" : ISODate("2017-05-31T10:23:18.265+04:00")
}

/* 3 */
{
    "_id" : ObjectId("592e7d182fc987735f3dda4f"),
    "subject" : "Test 2",
    "update" : ISODate("2017-05-31T10:23:18.266+04:00")
}

/* 4 */
{
    "_id" : ObjectId("592e7d182fc987735f3dda50"),
    "subject" : "Test 3",
    "update" : ISODate("2017-05-31T10:23:18.267+04:00")
}

// Sort "events.0.date" with "_id" allow disk
testcol.aggregate([
			{
                            $sort:{
                                "events.0.date": -1,
                                '_id':1
                            }
                        },
                        {
                               $limit:4
                         },
			{
                            $project:{
                            "subject":true,
                            "update": {
                                "$arrayElemAt": ["$events.date", 0]
                                }
                            }
                        }
                     ],
    {
        allowDiskUse: true
    })

//wrong result
/* 1 */
{
    "_id" : ObjectId("592e7d182fc987735f3dda4d"),
    "subject" : "Test 0",
    "update" : ISODate("2017-05-31T10:23:18.264+04:00")
}

/* 2 */
{
    "_id" : ObjectId("592e7d182fc987735f3dda4e"),
    "subject" : "Test 1",
    "update" : ISODate("2017-05-31T10:23:18.265+04:00")
}

/* 3 */
{
    "_id" : ObjectId("592e7d182fc987735f3dda4f"),
    "subject" : "Test 2",
    "update" : ISODate("2017-05-31T10:23:18.266+04:00")
}

/* 4 */
{
    "_id" : ObjectId("592e7d182fc987735f3dda50"),
    "subject" : "Test 3",
    "update" : ISODate("2017-05-31T10:23:18.267+04:00")
}



// Sort "update" (same as "events.0.date") with "_id"
testcol.aggregate([
			{
                            $sort:{
                                "update": -1,
                                '_id':1
                            }
                        },
                        {
                               $limit:4
                         },
			{
                            $project:{
                            "subject":true,
                            "update": {
                                "$arrayElemAt": ["$events.date", 0]
                                }
                            }
                        }
                     ])
//Good result
/* 1 */
{
    "_id" : ObjectId("592e80b52fc987735f4d1c8c"),
    "subject" : "Test 999999",
    "update" : ISODate("2017-05-31T10:39:58.263+04:00")
}

/* 2 */
{
    "_id" : ObjectId("592e80b52fc987735f4d1c8b"),
    "subject" : "Test 999998",
    "update" : ISODate("2017-05-31T10:39:58.262+04:00")
}

/* 3 */
{
    "_id" : ObjectId("592e80b52fc987735f4d1c8a"),
    "subject" : "Test 999997",
    "update" : ISODate("2017-05-31T10:39:58.261+04:00")
}

/* 4 */
{
    "_id" : ObjectId("592e80b52fc987735f4d1c89"),
    "subject" : "Test 999996",
    "update" : ISODate("2017-05-31T10:39:58.260+04:00")
}