From: "David A. Black" <dblack@...>
Date: 2005-11-29T07:50:26+09:00
Subject: Re: Question on class-declarations

Hi --

On Tue, 29 Nov 2005, Kris wrote:

> Yet Another Ruby Nuby (YARN) question.

Isn't Ruby Nuby redundant? :-)  But anyway, welcome.

> I see Rails makes extensive use of class-declarations.
>
> For example:
>
> class LineItem < ActiveRecord::Base
>  belongs_to :product
> ......
>
> belongs_to is a class-declaration in the above code. Coming from a C

It's not a declaration; it's a call to a method, specifically a class
method of ActiveRecord::Base.  (Actually, because of the way the
ActiveRecord code base is engineered, it's sort of a
pseudo-class-method, but I strongly advise you not to worry about
that :-)  For all practical purposes it's a class method.)

A class method is a method defined directly on a class object.  Here's
a simple example:

   class C
     def C.hello
       puts "hi"
     end
   end

   C.hello    # hi

Note that the class method C.hello is completely unrelated to C's
instance methods.  In fact, you could have an instance method with the
same name, with no conflict:

   class C
     def hello
       puts "hi from instance!"
     end
   end

   C.hello      # hi
   C.new.hello  # hi from instance!

> family of languages I could not grasp this initially. When does this
> code get executed - does it happen when the class is loaded for the
> first time or every time a new instance of the class is created?

It happens when the class definition itself is executed -- which will
be when the file is loaded.

> In this case I think the class-declaration code(belongs_to) looks at
> the corresponding database table structure and injects attributes and
> methods into the LineItem class. Am I right?

Yes.  The route to this is somewhat circuitous, but basically an
association method will trigger the matching up of table names,
thing_id fields, etc., and dynamically create the methods that allow
the object access to the associated class.

> I created the following example
>
> class B
> def hello
>   print "Hello"
> end
> end
>
> class D < B
>  hello
> def bye
>   print "bye"
> end
> end
>
> I tried this in irb and got an error. Why doesn't this work? In Rails I
> think mixins are used to provide this kind of functionality.

It doesn't work because hello and bye are both instance methods -- but
by calling hello in the definition scope of D, you're treating it like
a class method.  You're basically saying: send the message "hello" to
the class object D, which doesn't do anything because D doesn't
respond to that message.

> To me these seem to be idioms of the language. Is there a site where
> you could find gems like these?

I'm not quite sure what you mean... but I hope the above is helpful
:-)


David

-- 
David A. Black
dblack@wobblini.net