The java driver, by virtue of doing a null check on the `get("_id")` instead of a `containsField("_id")`, will try to set the `_id` to a new `ObjectId`. Not sure about all of the other drivers, but the javascript driver allows `null`. With respect to the use case of trying to replicate an existing collection, this causes a problem.

In DBCollection.java:

    public Object apply( DBObject jo , boolean ensureID ){

        Object id = jo.get( "_id" );
        if ( ensureID && id == null ){
            id = ObjectId.get();
            jo.put( "_id" , id );
        }

        doapply( jo );

        return id;
    }