Delete document from nested array in MongoDB

Question

I've created a collection with following insert statement

db.test.insert({
"_id": 1,
"updatedAt": ISODate("2016-05-27T05:37:44.928Z"),
"created_at": ISODate("2016-05-13T09:41:23.000Z"),
"batchId": "BATCH-Con-0",
"size": 7,
"currentlySubscribed": 0,
"startDate": ISODate("2016-05-15T18:30:00.000Z"),
"endDate": ISODate("2016-07-08T18:30:00.000Z"),
"dailyHours": 6,
"trainer": null,
"status": "Started",
"attendance": [
    {
        "date": ISODate("2016-05-04T18:30:00.000Z"),
        "students": [
            {
                "studentId": 1,
                "entryTime": ISODate("1969-12-31T19:31:00.000Z"),
                "exitTime": ISODate("1969-12-31T19:31:00.000Z"),
                "attendanceStatus": "Present"
            },
            {
                "studentId": 2,
                "entryTime": ISODate("1969-12-31T19:31:00.000Z"),
                "exitTime": ISODate("1969-12-31T19:31:00.000Z"),
                "attendanceStatus": "Present"
            },
            {
                "studentId": ("5735a3266c1e1dc423c302cf"),
                "entryTime": ISODate("1969-12-31T19:31:00.000Z"),
                "exitTime": ISODate("1969-12-31T19:31:00.000Z"),
                "attendanceStatus": "Present"
            }


        ]
    },
    {
        "date": ISODate("2016-05-05T18:30:00.000Z"),
        "students": [
            {
                "studentId": 1,
                "entryTime": ISODate("1969-12-31T19:31:00.000Z"),
                "exitTime": ISODate("1969-12-31T19:31:00.000Z"),
                "attendanceStatus": "Present"
            },
            {
                "studentId": 2,
                "entryTime": ISODate("1969-12-31T19:31:00.000Z"),
                "exitTime": ISODate("1969-12-31T19:31:00.000Z"),
                "attendanceStatus": "Present"
            },
            {
                "studentId": 3,
                "entryTime": ISODate("1969-12-31T19:31:00.000Z"),
                "exitTime": ISODate("1969-12-31T19:31:00.000Z"),
                "attendanceStatus": "Present"
            }
        ]
    }
    ]})

Now I want to delete the attendance record of some of the students with this query:

db.test.update({},{$pull: {"attendance.$.students": {"studentId":{$in:[1,2,3]}}}},{multi:true})

But this gives me an error stating "The positional operator did not find the match needed from the query. Unexpanded update: attendance.$.students"

Answer 1

If you want to use the $ positional operator, you have to have matched the array items already, something like this:

db.test.update(
  {"attendance.students.studentId":{$in:[1,2,3]}},
  {$pull: {"attendance.$.students": {studentId: {$in:[1,2,3]}}}},
  {multi:true}
)

Remember that this will only update the first matching item in the attendance array in each document, so you will have to run this update more than once to make it update each attendance item.

Answer 2

such a complex manipulations a very hard to achieve.

Workaround to that limitation is to use forEach loop on resultSet and interate every document

var searchCriteria = [1, 2, 3];
db.test.find({
    "attendance.students.studentId" : {
        $in : searchCriteria
    }
}).forEach(function (myDoc) {
    var len = myDoc.attendance.length;
    for (var i = 0; i < len; i++) {

        var x = myDoc.attendance[i];
        var xLen = x.students.length;
        var newArrayWithStudents = [];

        for (var j = 0; j < xLen; j++) {
            if (searchCriteria.indexOf(x[j].studentId) < 0) {
                newArrayWithStudents.add(x[j]);
            };

        };
        myDoc.attendance[i] = newArrayWithStudents;
    };

    myDoc.save();

});