[{"_id":ObjectId("6213ba90a013b7c5f1232e1f"),"name":"name1","surname":"surname1","newArray":{"buyDate":ISODate("1975-11-04T13:14:15Z"),"carCount":3,"name":"name1","surname":"surname1"}},{"_id":ObjectId("6213ba90a013b7c5f1232e2f"),"name":"name2","surname":"surname2","newArray":{"buyDate":ISODate("1978-12-07T15:30:15Z"),"carCount":7,"name":"name2","surname":"surname2"}},{"_id":ObjectId("6213ba90a013b7c5f1232e3f"),"name":"name3","surname":"surname3","newArray":{"buyDate":ISODate("1983-11-12T18:40:15Z"),"carCount":4,"name":"name3","surname":"surname3"}}]

db.collection.aggregate([{$set:{"newArray.buyMonth":{$month:"$newArray.buyDate"}}},{"$group":{"_id":"$newArray.buyMonth","carCountByMonth":{"$sum":"$newArray.carCount"}}}])