>  My question is: how can I control in this type of application instances
of
>  Transaction? Perhaps with a class variable in Transaction? Any other
great
>  idea? What do you think of it?.

I'm not sure what you mean by "control ... instances of Transaction"; can
you be more specific?

If you're trying to limit the number of Transaction instances that can be
created, you might want to consider the "Singleton" pattern:

    http://www.rubycentral.com/book/lib_patterns.html

with some modifications if you don't want to limit yourself to exactly one
transaction instance.

If you're trying to ensure that all of the GUI windows associated with a
transaction maintain a consistent state, you might want to consider the
"Observer" design pattern (also discussed on the same page listed above).
These are the Ruby library's implementation of these popular design
patterns, but for the original source (which provides more background about
the motivation, etc.) you should see the book "Design Patterns", by Erich
Gamma and friends.

Hope this helps,

Lyle