David Corbin wrote:

> You're right, that's not what I want.  What I really want, now that I 
> thiink about it, is for objects to be loaded "on-demand", and for 
> dirty objects to be periodically "flushed to disk". 

That sounds to me like an OO database. Which does exactly that and it's 
_really_ nice for the application programmer.
Unfortunately the only decent one I know is GOODS and that has no Ruby 
interface yet. There is a perl interface for the bravehearted "porter".
Goods does exactly this Schema evolution and copes with several clients 
using different schemas. I've worked with only from java, and it's 
really nice, so just for information, here's a URL 
http://www.garret.ru/~knizhnik/goods.html.

Torsten