Upserts in mongodb when using custom _id values

Question

I need to insert a document if it doesn't exist. I know that the "upsert" option can do that, but I have some particular needs.

First I need to create the document with its _id field only, but only if it doesn't exist already. My _id field is a number generated by me (not an ObjectId). If I use the "upsert" option then I get "Mod on _id not allowed"

db.mycollection.update({ _id: id }, { _id: id }, { upsert: true });

I know that we can't use the _id in a $set.

So, my question is: If there any way to a "create if doesn't exists" atomically in mongodb?

EDIT: As proposed by @Barrie this works (using nodejs and mongoose):

var newUser = new User({ _id: id });
newUser.save(function (err) {               
    if (err && err.code === 11000) {            
            console.log('If duplicate key the user already exists', newTwitterUser);
        return;
    }
    console.log('New user or err', newTwitterUser);
});

But I still wonder if it is the best way to do it.

Answer 1

I had the same problem, but found a better solution for my needs. You can use that same query style if you simply remove the _id attribute from the update object. So if at first you get an error with this:

db.mycollection.update({ _id: id }, {$set: { _id: id, name: 'name' }}, { upsert: true });

instead use this:

db.mycollection.update({ _id: id }, {$set: { name: 'name' }}, { upsert: true });

This is better because it works for both insert and update.