From: zig <ziggurism@...>
Date: 2007-03-25T06:50:08+09:00
Subject: class methods and instance variables in ActiveRecord::Base

I'm still learning my way around ruby and I seem to be confused about
class methods/variables versus instance methods/variables.  My example
comes from Ruby on Rails, but I don't think my confusion is rails-
specific.

In ActiveRecord::Base, there is a method columns.  I initially thought
this was an instance method since it wasn't declared as
ActiveRecord::Base.columns, which is the required syntax for class
methods.  However someone explained to me that you can enclose several
method declarations in a class << self block which makes them all
class methods.

OK, so columns is a class method.  But then in its source, it makes
reference to the instance variable @columns.  In other OOP languages
that I'm familiar with, it's forbidden to access instance variables
within a class method (if the class hasn't been instantiated, then the
instance variables can't have been initialized).  I've investigated
the situation in ruby, and apparently it is this: it's not illegal to
reference an instance variable from within a class method, but the
instance variable will be nil.

In the rails scaffolding, the code calls Modelname.columns.  So it's
calling the class method, and the class has not been instantiated.

Help de-confuse me!  How does this class method get away with
referencing an instance variable and where exactly does this instance
variable get initialized? (I can't find any occurrences of @columns
outside of other class methods, so it's apparently not being
initialized anywhere in this class?)

Thanks in advance,
zig