Mongoose: Using addToSet with ObjectIds Results in Orphan Id - node.js

Question

I am having a rather interesting problem using mongoDB's $addToSet to an array full of ObjectIds.
In my mongoose schema ("Happening"), I declare an array of ObjecIds called "expected", to be used by .populate().
expected: [{type: Schema.Types.ObjectId, ref: "User" }]
... which works nicely everywhere I use it. So far so good.
I then attempt to update the Happening.expected array using $addToSet as outlined here:
http://docs.mongodb.org/manual/reference/operator/addToSet/
like so:
app.get("/happening/yamobethere/:id", ensureLoggedIn("/login"),
function (req, res) {
// userId is the mongo ObjectId of the user record
var userId = req.session.user.id,
eventId = req.params.id;
models.Happening.update(
{_id: eventId}, {
$addToSet: {expected: userId}
},
function(err, updated){
if (err) {
res.json({"error": err});
}
res.json({"updated": updated});
});
});
... which always yields:
{updated: 1}
Now the docs lead me to expect the actual userId that I passed in, so the "1" is a bit odd. I expected it to be a fail, and in light of the weirdness that happens next, it appears to be a mongodb error of some sort percolating it's way back to me as results.
The weirdness is, when I check my database, I see that indeed a new ObjectId has been added: just not the one I passed in.
"expected" : [
ObjectId("51cb18623ade2b9f1e000004"),
ObjectId("51cdb7c12f0e58bdb3000001")
],
becomes
"expected" : [
ObjectId("51cb18623ade2b9f1e000004"),
ObjectId("51cdb7c12f0e58bdb3000001"),
ObjectId("51cdb80e09612bfab3000002")
],
The new ObjectId does not appear in any of my collections. It appears to be an orphan, but I'm a mongo noob, so I may be full of compost on this.
I did attempt to cast the userId as an ObjectId:
$addToSet: {expected: mongoose.Types.ObjectId.fromString(userId)}
but that changed nothing, and really should not be necessary, since the schema should handle it.
I'd really rather not resort to downloading the entire object, appending the value to the "expected" array, then sending the whole schmear back for an update.
Any help appreciated, folks. Thanks!
Update:
A colleague suggested the following technique:
var addMe = {$addToSet: {expected: userId}};
models.Happening.findByIdAndUpdate(eventId, addMe, function(err, me) {
if (err) {
return json(err);
}
res.json(200, me);
});
... which is a bit of an improvement, since it actually returns an object for me to inspect. Unfortunately, it also results in orphaned ObjecIds appearing in the array, rather than the existing userId value I specified.
Thanks again!

Answer 1

It appears that my passport strategy is returning the ObjectID of the rejected attempted creation of a new user in the db via data from oauth. So, the code is fine, my data is garbage.
Never trust anything, and be prepared to look like a boob. :-)
Thanks for the clarification on my return values JohnnyHK.