From: "Luke A. Kanies" <luke@...>
Date: 2004-08-13T11:37:23+09:00
Subject: Relationship between classes

Hi all,

I think this is a ruby question, as my problem is written in ruby, but it 
is mostly just a programming question.

I have two types of classes, objects and operations.  Each object has one 
or more operation class associated with it (meaning that these are 
operations which can be executed on the objectgs).  When an object 
instance is created, it is created with one or more attributes, and as 
many operations as can be successfully run will also be created.

For instance, given a File operation, and chmod, chown, and chgrp 
operations, if I create a File instance with only ownership information, 
then I can only create an instance of the chmod operation.

Now, to the problem:  I am have serious difficulty coming up with a good 
way to store these relationships.  It's a one-to-many, in that each object 
class is related to many operations.  It would be easy if I were willing 
to just let the operations call the objects directly:

chmod(file.path, file.owner)  # or whatever

But because some operations might be associated with more than one object 
(e.g., chmod would be associated with both Files and Directories), I can't 
afford to assume that much about the objects.  What I want to do is 
somehow store the association information with the object, like this:

...
                 :chown => [
                     [:owner, :file],
                     proc { |object|
                         Blink::Operation::Chown.new(
                             :owner => proc { |object| object[:uid] },
                             :file => proc { |object| object[:file] }
                         )
                     }
                 ],
...

This stores my three key pieces of information:  What the object calls the 
operation (so it knows how to execute it on itself later), what attributes 
are necessary in order to create an instance of the operation, and how to 
actuallly instantiate the operation.

I am currently storing all of these in a big hash in each object class 
definition.

This seems really inelegant, though, and it causes some weird side effects 
in my thinking.

Is there some better way to store these relationships, without causing 
leaking of the object interface into the operation definition?

Much thanks,
Luke

-- 
Commit suicide. A hundred thousand lemmings can't be wrong.
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Luke Kanies | http://abstractive.org | http://reductiveconsulting.com