From: Siemen Baader <sbaader@...>
Date: 2007-03-28T07:50:04+09:00
Subject: dynamically adding class variables

Hi,

I am writing an object relational mapper for an educational project 
where I need to add new class variables to classes in an inheritance 
hierarchy. The variables hold class-specific information such as  which 
tables are mapped and validation logic so they cannot be shared among 
the hierarchy like inherited class variables from the superclass would. 
Because of that I could not declare them in the parent and just 
initialize them in the subclasses. Instead, I decided to add and set 
them at runtime after subclassing.

However, i think ruby behaves a bit odd here. In class methods inherited 
from the superclass I can see the class variables through the method 
class_variables, but to use them in the child class I have to re-open 
the children and re-define the class methods which use the variables!


?> class ORM
 >>   def ORM.table table
 >>     class_eval("@@table = :%s" % table)
 >>   end
 >>
?>   def ORM.vars
 >>     class_variables
 >>   end
 >>   def ORM.table?
 >>     @@table
 >>   end
 >> end
=> nil
 >>
?> class Article < ORM
 >>   table :articles
 >> end
=> :articles
 >>
?> Article.vars
=> ["@@table"]
 >> Article.table?
NameError: uninitialized class variable @@table in ORM
         from (irb):22:in `table?'
         from (irb):31
         from :0
 >>
?> class Article
 >>   def Article.table?
 >>     @@table
 >>   end
 >> end
=> nil
 >>
?> Article.table?
=> :articles

Could anyone enlighten me on why class_variables finds the class 
variable in the subclass and why the class methods act like they were 
invoked on the parent class? Apparently class_variables finds the 
variables in a different way than ruby does when it evaluates the method 
code...

Also, has anyone a good idea how I can give each child class variables 
which are shared among all instances of the classes but isolated from 
parents and other children?

thanks,
siemen baader