From: James Edward Gray II <james@...>
Date: 2005-11-07T06:33:24+09:00
Subject: Re: Basic help with understanding RoR Inheritance

On Nov 6, 2005, at 2:42 PM, TomRossi7 wrote:

> I'm sorry for the newbie question, but I can't seem to find a basic
> enough explanation for what I am looking for.  Basically, I am trying
> to understand Ruby on Rails Single Table Inheritance and the required
> database structure for easy ActiveRecord integration.  Here is a  
> simple
> example that I would be very grateful for assistance:
>
> Imagine you want to create an application built for tracking vehicle
> information.  You have a super-class Vehicle with attributes like VIN,
> manufacturer, manufacturer date etc. -- all of the attributes that are
> shared by all vehicles.  Then you have sub-classes like car, truck,
> motorcycle, etc.  These sub-classes have unique attributes like  
> maybe a
> "towing capacity"  for trucks [Granted, the attributes aren't that
> dissimilar between a car and a truck, but this is just an example].
>
> So, how would this best be modeled in the database and RoR?  I am
> thinking a model (class) for Vehicle, Car, Truck, Motorcyle.  The
> tables would be vehicles, cars, trucks, motorcylces.  They each would
> have id's and the vehicle table would track for each id the "type" of
> vehicle.
>
> Am I on the right track?

Close.  Remember it's called "Single Table" Inheritance.  That means  
you only make a "vehicles" table, and add a "type" field.   
ActiveRecord pretty much handles the rest.

There's information on this in the Rails book, in case you want to  
know more.

James Edward Gray II