[{"_id":{"$oid":"608378ef1ae6b1368c6220c6"},"username":"paul","password":"cacavaca123","question":"q1","answer":"cutu"},{"_id":{"$oid":"6084c1b557f2242bcc629440"},"username":"mircea","password":"123456","question":"q1","answer":"cutu"}]

db.collection.aggregate([{$match:{"username":"paul"}},{$set:{friends:["alex"]}}])