db={"user":[{"_id":"6260267ce449a8c74cf77ef7","email":"admin@admin.com","password":"####","name":"admin","unreadMessages":[{"_id":"643dbcca2329d2bf497977a2"}]},{"_id":"6260267ce449a8c74cf77ef8","email":"eelco@admin.com","password":"####","name":"eelco","unreadMessages":[{"_id":"643dbcca2329d2bf497977a6"}]}],"customermessages":[{"_id":"643dbcca2329d2bf497977a2","name":"User","messages":[{"message":"Hello, I am interested in this project","refrenceName":"x123","_id":"643dbcca2329d2bf497977a5",}]},{"_id":"643dbcca2329d2bf497977a3","name":"User","messages":[{"message":"Hello, I am interested in this project","refrenceName":"x124","_id":"643dbcca2329d2bf497977a6",}]}]}

db.user.aggregate([{$lookup:{from:"customermessages",localField:"unreadMessages._id",foreignField:"_id",as:"unreadMessages",},},{$unwind:"$unreadMessages",},{$group:{_id:"$_id",email:{$first:"$email"},password:{$first:"$password"},name:{$first:"$name"},unreadMessages:{$push:"$unreadMessages"},},},])