findOneAndUpdate works part of the time. MEAN stack - node.js

Question

I'm working with the mean stack I'm trying to update the following object:
{
_id : "the id",
fields to be updated....
}
This is the function that does the updating:
function updateById(_id, update, opts){
var deferred = Q.defer();
var validId = new RegExp("^[0-9a-fA-F]{24}$");
if(!validId.test(_id)){
deferred.reject({error: 'invalid id'});
} else {
collection.findOneAndUpdate({"_id": new ObjectID(_id)}, update, opts)
.then(function(result){
deferred.resolve(result);
},
function(err){
deferred.reject(err);
});
}
return deferred.promise;
}
This works with some of my objects, but doesn't work with others.
This is what is returned when it fails to update:
{
ok: 1,
value:null
}
When the function is successful in updating the object it returns this:
{
lastErrorObject: {}
ok: 1
value: {}
}
It seems like Mongo is unable to find the objects I'm trying to update when it fails. However, I can locate those objects within the Mongo shell using their _id.
Does anybody know why the driver would be behaving this way? Could my data have become corrupt?
Cheers!

Answer 1

I found the answer and now this question seems more ambiguous so I apologize if it was confusing.
The reason I was able to find some of the documents using ObjectID(_id) was because I had manually generated some _id fields using strings.
Now I feel like an idiot but, instead of deleting this question I decided to post the answer just in case someone is running into a similar issue. If you save an _id as a string querying the collection with the _id field changes.
querying collection with MongoDB generated _ids:
collection.findOneAndUpdate({"_id": new ObjectID(_id)}, update, opts)
querying collection with manually generated _ids:
collection.findOneAndUpdate({"_id": _id}, update, opts)
In the second example _id is a string.
Hope this helps someone!