Remove getRouteKeyName from Model

I am in the process of trying to clear up a variety of little aspects of a project I inherited and am in the process of removing the odd Route Key Names from the models. I am testing with the easiest model which had the following in it:

public function getRouteKeyName(){
        return 'url_string';
    }

I just deleted the lines and saved it but unfortunately it returned "Sorry this page could not be found" when using the regular ID, but brought up successfully using the field contents of url_string. What else do I have to do to remove its dependence on the url_string field?

Thanks! Matt

via Chebli Mohamed