How to delete the documents returned by group in mongodb? -

i mongodb beginner , working on homework problem, dataset looks this

{ "_id" : { "$oid" : "50906d7fa3c412bb040eb577" }, "student_id" : 0, "type" : "exam", "score" : 54.6535436362647 } { "_id" : { "$oid" : "50906d7fa3c412bb040eb578" }, "student_id" : 0, "type" : "quiz", "score" : 31.95004496742112 } { "_id" : { "$oid" : "50906d7fa3c412bb040eb579" }, "student_id" : 0, "type" : "homework", "score" : 14.8504576811645 } { "_id" : { "$oid" : "50906d7fa3c412bb040eb57a" }, "student_id" : 0, "type" : "homework", "score" : 63.98402553675503 } { "_id" : { "$oid" : "50906d7fa3c412bb040eb57b" }, "student_id" : 1, "type" : "exam", "score" : 74.20010837299897 } { "_id" : { "$oid" : "50906d7fa3c412bb040eb57c" }, "student_id" : 1, "type" : "quiz", "score" : 96.76851542258362 } { "_id" : { "$oid" : "50906d7fa3c412bb040eb57d" }, "student_id" : 1, "type" : "homework", "score" : 21.33260810416115 } { "_id" : { "$oid" : "50906d7fa3c412bb040eb57e" }, "student_id" : 1, "type" : "homework", "score" : 44.31667452616328 } 

as part of problem have delete each student, 'homework' document lowest score. here strategy

in aggregate pipeline
1: first filter documents type:homeworks
2: sort student_id, score
3: group on student_id, find first element

this give me documents lowest score,

however how delete these elements original dataset? guidance or hint?

use cursor result aggregation loop through documents cursor's foreach() method , remove each document collection using _id query in remove() method. this:

var cursor = db.grades.aggregate(pipeline); cursor.foreach(function (doc){     db.grades.remove({"_id": doc._id}); }); 

another approach create array of document's _ids using map() method , remove documents like:

var cursor = db.grades.aggregate(pipeline),     ids = cursor.map(function (doc) { return doc._id; }); db.grades.remove({"_id": { "$in": ids }}); 

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-- update --

for large deletion operations, may more efficient copy documents want keep new collection , use drop() on original collection. copy essential documents aggregation pipeline needs return documents without lowest homework doc , copy them collection using $out operator final pipeline stage. consider following aggregation pipeline:

db.grades.aggregate([         {         '$group':{             '_id': {                 "student_id": "$student_id",                 "type": "$type"             },             'lowest_score': { "$min": '$score'},             'data': {                 '$push': '$$root'             }          }     },         {         "$unwind": "$data"     },     {         "$project": {             "_id": "$data._id",             "student_id" : "$data.student_id",             "type" : "$data.type",             "score" : "$data.score",             'lowest_score': 1,                         "ishomeworklowest": {                 "$cond": [                     {                          "$and": [                             { "$eq": [ "$_id.type", "homework" ] },                             { "$eq": [ "$data.score", "$lowest_score" ] }                         ]                      },                     true,                     false                 ]             }         }     },     {         "$match": {"ishomeworklowest" : false}     },     {         "$project": {                        "student_id": 1,             "type": 1,             "score": 1         }     },     {         "$out": "new_grades"     } ]) 

in can drop old collection db.grades.drop() , query on db.new_grades.find()