MongooseJS Not saving to array properly

I want to append a value into my Mongoose array but my array never seems to update. I do the following:

In my controller, I append an eventName into the array eventsAttending like so:

  $scope.currentUser.eventsAttending.push(event.eventName);
  $http.put('/api/users/' + $scope.currentUser._id, $scope.currentUser)
    .success(function(data){
      console.log("Success. User " + $scope.currentUser.name);
    });

I try to update the array like so:

// Updates an existing event in the DB.
exports.update = function(req, res) {
  if(req.body._id) { delete req.body._id; }
  User.findById(req.params.id, function (err, user) {
    if (err) { return handleError(res, err); }
    if(!user) { return res.send(404); }
    user.markModified('req.body.eventsAttending');
    user.save(function (err) {
      if (err) { return handleError(res, err);}
      return res.json(200, user);
    });
  });
};

But my array never seems to update. I've also tried the following:

// Updates an existing event in the DB.
exports.update = function(req, res) {
  if(req.body._id) { delete req.body._id; }
  User.findById(req.params.id, function (err, user) {
    if (err) { return handleError(res, err); }
    if(!user) { return res.send(404); }
    var updated = _.merge(user, req.body);
    updated.markModified('eventsAttending');
    updated.save(function (err) {
      if (err) { return handleError(res, err);}
      return res.json(200, user);
    });
  });
};

With this approach, my array updates properly, but when I try to perform the http put after one time, I get an error saying Error: { [VersionError: No matching document found.] message: 'No matching document found.', name: 'VersionError' }

Here is my UserSchema:

var UserSchema = new Schema({
  name: String,
  username: String,
  eventsAttending: [{ type: String, ref: 'Event'}],
});

If anyone could help that would be much appreciated.