Changing response doc object from findOneAndUpdate

Say I got this lil chunk of code:

 Room.findOneAndUpdate({ Roomid: roomid }, { $push: { UsersMeta: UserMeta}}, { new: false }, function (err, room) {
    if (err) console.log(err);
    console.log('room output:');        
    console.log(room);
    client.emit('others', room);
 })  

which is searching for one document in db, updates it, and then sends this room doc in pre-updated state back to client. What I need is to make some changes to responded room, particularly remove those _id, __v, and, possibly, any other custom part of doc.

What I was trying to do:

use toObject.transform while creating schema

var RoomSchema = mongoose.Schema({
Roomid: { type: String, unique: true },
///stuff///
});

RoomSchema.options.toObject.transform = function (doc, ret, options) {
// remove the _id of every document before returning the result
delete ret._id;
}

failed: recieved cannot set property 'transform' of undefined error.

Change mentioned chunk of code to:

 Room.find({ Roomid: roomid })
.update({ $push: { UsersMeta: UserMeta} })
.select({ _id: 0 })
.exec(function (err, room) {
    if (err) console.log(err);
    console.log('room output:');
    console.log(room);
    client.emit('others', room);
})

Failed: always recieve [] in room output.

Now I stopped at manually setting {_id: false} on Schema declaration, completely getting rid of _id in the first place. As I want use custom random id's for rooms, it seems that I don't need those _id anyway. Yet I'm not sure, that such a treatment will not cause some unpleasant consequences.

And, moreover, problem of possible need to leave some non _id doc properties out of response - is an unsolved mystery for me.

Thank you for attention.