Noteboard Engine The engine inherits from the host app Application Controller and thus depends on a host app for testing and use. There needs to a be a User class within the host app because the engine uses the User ID to display a username instead of poster_id and a link to the home page. The User class is also used in the Note validation to make sure the poster_id is a real user. The Noteboard itself has an association with the User class. There is also a method myRenderer in the host app application controller that the engine uses to handle json. The engine inherits its layout from the host app.