I want to add object with CERTAIN _id if object doesn't exist. Else, I want to use $set on it's properties.
What should I do?

> db.chatsessions.update(

{"_id": "test"}

,

{"_id":"test"}

,

{upsert: true}

);
> db.chatsessions.find();

{ "_id" : "test" }

<_id": "test1"},{"_id":"test1","$set":{"key": "value"}},

{upsert: true}

);
Modifiers and non-modifiers cannot be mixed
> db.chatsessions.update(

{"_id": "test1"}

,{"$set":{"_id": "test1", "key": "value"}},

{upsert: true}

);
Mod on _id not allowed
>

I think that the error "Modifiers and non-modifiers cannot be mixed" shouldn't occur in case of _id.
In case that an object exists already, _id should be ignored.