From: Thomas Hurst <tom.hurst@...>
Date: 2002-05-20T02:45:43+09:00
Subject: Re: Object <-> Relational mapping

* Chris Morris (home@clabs.org) wrote:

> > db.select_all(long_sql_statement) do |row|
> >         foo = Foo.new
> >         foo.a = row['a']
> >         foo.b = row['b'].to_i
> >         foo.c = row['c'].to_f
> >         #...
> >         foo.z = row['z']
> > end

> Some possible improvements. I'd try to take advantage of .send - so
> the loop would be something like:

> ... do |row|
>   # row would need to be a hash
>   row.each do |colname, colvalue|
>     foo.send('=' + colname, colvalue)
>   end
> end

Unfortunately, my row names do not map directly to object properties, be
it because the word I want to use is reserved, or whatever.

I could map it I guess, using a hash.

> As far as handling row['b'].to_i, etc. -- if it has translate types,
> make that transformation in the set accessor method of foo's class.

Yes, probably a good general idea.

This still doesn't help dealing with relations or make the SQL less
painful.  Each object is quite likely to have some objects related to
it; that requires additional logic watching for id changes and building
new objects and maybe adding to them inside the object, ala:

last_id = nil
db.select_all(bla) do |row|
        if last_id == row['Id'].to_i
                bar = FooBar.new
                # build bar using whatever method
                foo.collection << bar
        else
                records << foo
                last_id = row['Id'].to_i
                foo = Foo.new
                # build foo using whatever method
        end
end

Some of this can probably be delegated to superclasses quite easily, but
you can see it's still tightly coupling the db schema (need to abstract
away the SQL) and the object's makeup in relation to it.

[automatically generating src]
> You could probably do the same for Ruby classes (including
> automatically coding for translations from string to the actual column
> datatype).

hm, could do..

<starts reading http://www.ambysoft.com/persistanceLayer.pdf>

> I don't know of any generic mapping lib efforts -- but I'd probably be
> able to contribute some as needs arise at work.

One was with some application server iirc, probably around
ruby-talk:25000

-- 
Thomas 'Freaky' Hurst  -  freaky@aagh.net  -  http://www.aagh.net/
-
Political history is far too criminal a subject to be a fit thing to
teach children.
		-- W.H. Auden