MongooseJS modify document during pre hook

ArticleSchema.pre('save', function(next) {

    this.categories = _.map(this.categories, function(category) { 
        console.log(category.toLowerCase());
        return category.toLowerCase();  
    });    
    next();
});

var User = mongoose.model('User');

TopicSchema.pre('save', function(next) {
  var self = this;
  User.findById( self.user, function (err, user) {
    if (err) // Do something
    var usersTime = moment().tz(user.timezone).hours(0).minutes(0).seconds(0).milliseconds(0);  // Reset the time to midnight
    var nextNotifyDate = usersTime.add(1, 'days').seconds(self.timeOfDay);                       // Add a day and set the reminder
    self.nextNotificationDate = nextNotifyDate.utc();

    next();
  });
});