How to change the document using pre save in Mongoose

I am trying to assign the pre handler to the mongoose save event and encrypt the document before saving:

userShecma.pre('save', function(next) {
    var self = {};
    self.Key = this.password;;
    self.EncriptedString = encrypt.encrypt(JSON.stringify(this), this.password);
    self.user = this.user
    self.decrypt = function() {
        var user = JSON.parse(encrypt.decrypt(this.EncriptedString, this.Key));
        for(var key in user) {
            this[key] = user[key];
        }
    }
    for(var key in this){
        delete this[key];
    }
    for(var key in self){
        this[key] = self[key];
    }
    console.log(this);
    next(self);
});

I have tried a bunch of diffrent things, sometimes I get an error, sometimes it just doesnt change the document.

Let me know if you need any more info,
Ari

EDIT: Tried Benoir's Answer, I can't edit this.