From: dblack@...
Date: 2002-09-23T23:25:45+09:00
Subject: Re: self as method argument revisited

Hi --

On Mon, 23 Sep 2002, Christoph wrote:

> dblack wrote
> ....
> > A while ago transami brought up the question of an object needing
> > access to the object that created it, and there was discussion of
> > techniques including sending self as an argument.  I've been working
> > on a case of the underlying problem he was talking about, and I
> > thought I'd share my (current) solution.  Comments welcome.
> >
> > The big object is a (simple) database image.  It creates Row objects
> > from arguments to an #insert method, and populates itself hash-wise
> > with the values.
> >
> > There are also field names.  A Row object needs to know its own field
> > names, so that one can do things like:
> >
> >   r = db["Black"]   # r is a Row object
> >   r["first_name"]   # => David
> >
> > Originally I had been creating an array of field names for every row,
> > but that seemed very clunky.  Then I got an idea....
> >
> > When the database object is created, it creates a new class, which
> > inherits from an existing Row class (DBDBD::Row).  That new class has
> > an instance variable (i.e., its virtual class defines an accessor
> > method) called @row_class.  The new database object assigns itself to
> > @db in that Row class's virtual class:
>
> While I am still marveling what problem your are trying to solve -
> I am sort of wandering why you are creating a Row constant in
> the scope of the meta  (virtual) class of every DBDB instance?
> I doubt hat you woould ever make use of this constant, unless
> your are planning on peppering every DBDB instance with a
> barrage of singleton methods.

Don't marvel -- I may well be on the wrong track :-)  I'm not sure I
understand what you're saying, though.  My thinking is: each database
(DBDB instance) needs to create rows, and those rows need to know how
to send messages to the database.  Since I don't want to put that
knowledge in a static Row class (since different databases will have
different fields), I thought it would make sense to have a separate
Row class for every database.

>
> ---
>  class DBDB # The 1.6 object model is inferior
>     def initialize(...)
>       ...
>       @row_class = Class.new (DBDBD::Row)
>       class << @row_class
>          attr_accessor :db
>       end
>       @row_class.db = self
>     end
>     ...
>  end
>
>
> class DBDB  # 1.7 version
>    class Row
>       ...
>       @row_class = self
>        class << self
>           attr_accessor :db
>        end
>    end
>    def initialize(...)
>       ...
>       @row_class = Class.new (DBDBD::Row)
>       @row_class.db = self
>    end
>     ...
>  end

In your next post you change that to @row_class.db = nil.  But isn't
self correct?  The idea is for the Row class's db attribute to contain
a reference to this data base object.


David

-- 
David Alan Black                      | Register for RubyConf 2002!
home: dblack@candle.superlink.net     | November 1-3
work: blackdav@shu.edu                | Seattle, WA, USA
Web:  http://pirate.shu.edu/~blackdav | http://www.rubyconf.com