Upserts in mongodb when using custom _id values - mongodb

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.

Answer 2

UPDATE: Upsert with _id can be done without $setOnInsert, as explaind by #Barrie above.
The trick is to use $setOnInsert:{_id:1} with upsert, that way the _id is only written to if it's an insert, and never for updates.
Only, there was a bug preventing this from working until v2.6 - I just tried it on 2.4 and it's not working.
The workaround I use is having another ID field with a unique index. Eg. $setOnInsert:{myId:1}.

Answer 3

You can just use insert(). If the document with the _id you specify already exists, the insert() will fail, nothing will be modified - so "create if it doesn't exist" is what it's already doing by default when you use insert() with a user-created _id.

Answer 4

Please note that $setOnInsert don't work easily when you upsert a simple key => value object (not $set or other).
I need to use that (in PHP):
public function update($criteria , $new_object, array $options = array()){
// In 2.6, $setOnInsert with upsert == true work with _id field
if(isset($options['upsert']) && $options['upsert']){
$firstKey = array_keys($new_object)[0];
if(strpos($firstKey, '$')===0){
$new_object['$setOnInsert']['_id'] = $this->getStringId();
}
//Even, we need to check if the object exists
else if($this->findOne($criteria, ['_id'])===null){
//In this case, we need to set the _id
$new_object['_id'] = $this->getStringId();
}
}
return parent::update($criteria, $new_object, $options);
}