Reservation polymorphism

[nhocki]

As seen on Gist: https://gist.github.com/4280095

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I have a small problem. I'm trying to build a ruby client for a remote lab system
called Weblab Deusto. The basic flow of running an experiment on a lab is simple,
you have a username/password that you use to login. Once logged in, you can
access a list of experiments, each of those having a name and a category that
combined form a unique identifier inside the lab system.

Having an experiment, you can reserve that experiment and here's where things
get a little bit messy. The reserve response from the server will have
a status attribute. Depending on the status of the reservation, you will get
some parameters. There are five different status for a reservation.

All the clients I've seen implement this by having 5 different classes, one for
each status. Then, depending on the response, they instantiate one of those. I
feel this to be a little messy but I have no idea on how to make this better.

The main reason I think this approach is messy is because, everytime I'm going
to use a reservation (for any purpose), I'd have to ask what kind of reservation
it is, since their public interface is different. Let's think about a reservation
that is currently on position 50 on the queue, and I want to check the new
position of it. The problem is that if the status has changed, the reservation
may have no possition (because it's already finished for example). This way, I
can't do something like:

reservation.update_status!.position

I would have to do something like:

reservation.update_status!.respond_to?(:position)

I've thought about having all the attributes (like position) in all the reservation
classes, and that way I will have the same interface on all the classes, but I
have no idea if this is even worse than the current approach in other clients.

Any help? Thanks!

• Weblab Deusto: https://www.weblab.deusto.es/web/
• Weblab Deusto Clients: https://github.com/porduna/weblabdeusto/tree/master/server/consumers
• Ruby Client I'm trying to build: https://github.com/lms4labs/deustorb