Cannot pull from mongoose array - node.js

Question

In my schema I have two arrays with users, invitedUsers and joinedUsers. Invited array is created from request body
invitedUsers: req.body.invitedUsers
Joined array is updated when user clicks 'Join'
$addToSet: {joinedUsers: req.user}
I can then pull user from joinedUsers array by performing
{$pull: {joinedUsers: {_id: req.user._id}}}
However, I can't pull from invited users array, although user object is the same. Here is the complete code:
app.put('/decline/events/:id', function (req, res) {
var update = {$pull: {invitedUsers: {_id: req.user._id}}};
console.log(req.user._id);
Event.findByIdAndUpdate(req.params.id, update, function (err, event) {
if (err) {
res.send(err);
};
Event.find({})
.populate('owner')
.exec(function (err, events) {
if (err) {
res.send(err);
};
res.json(events)
});
});
});
Ajax call on client
$scope.declineInvitation = function (id) {
$http.put('/decline/events/' + id)
.success(function (data) {
})
.error(function (data) {
console.log('Error: ' + data);
});
};
Everything goes without errors, but invitedArray doesn't change, as if mongoose fails to find it. What might be a solution to this?
Update
Event Schema definition, as requested:
var eventSchema = new Schema({
title: {type: String},
description: {type: String, default: ''},
startDate: Date,
invitedUsers: {type: Array, default: []},
joinedUsers: {type: Array, default: []},
owner: {type: Schema.Types.ObjectId, ref: 'User'},
created_at: {type: Date, default: Date.now()},
updated_at: {type: Date, default: null},
location: String
}

Answer 1

The problem was because the _id in invitedUsers is a String and req.user._id is an ObjectId.
Calling toString() on _id will solve it:
var update = {$pull: {invitedUsers: {_id: req.user._id.toString()}}};