Re: setting up relationships

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On Mon, Jun 21, 2004 at 12:30:58 -0700, steve shapero wrote:
> hello list--

Hi!

> i am having some trouble setting up relationships for my database, which
> has a fair number of "lookup" tables.  given these tables, i'm wondering
> how i can setup a relationship between them.  also, any advice on how to
> use AutoLoader in this scenario would be great.

Class::DBI::Loader looks at your schema, and then creates some packages
for you.

Read the synopsis of Class::DBI::Loader [1]. In your case, using

	my $loader =3D Class::DBI::Loader->new(
		dsn =3D> $dsn,
		namespace =3D> 'C'
	);

you will be able to ask $loader what class was generated for the artist
table:

	$loader->find_class('artist');

The class it will return would have been already loaded, so just doing

	C::Artist->retrieve(1);

should have worked.

Class::DBI::AutoLoader is used a bit differently - it generates the
namespaces, period. There is no loader object, which you can harass. The
implementations are different though, and it looks like
Class::DBI::Loader is a little more robustly designed.

Neither of these sets up relationships. For this you have to do some
manual labor...

> in this scenario, a track has n number of artists, and an artist might
> have produced x number of tracks...

This is solved by mapping.

I'll assume you used CDBI::Loader with a namespace of 'C'.

Lets start with the lookup table:

	package C::Track_Artsit; # [sic]
	__PACKAGE__->has_a( artist =3D> 'C::Artist' );
	__PACKAGE__->has_a( track =3D> 'C::Track' );


This definition of the relationship will let you take an object of class
C::Track_Artsit, lets call it $link, and know the associated artist and
track of that link:

	$link->artist; # returns a C::Artist->retrieve() on the
	               # value of the 'artist' column

This basic relationship is then used by a mapping:

	package C::Artist;
	__PACKAGE__->has_many( tracks =3D> [ 'C::Track_Artsit' =3D> track ] );

What's happenning here is that an artist has many C::Track_Artsit. When
you call tracks, it retrieves all the C::Track_Artsit objects whose
'artist' column matches the primary key of an $artist.

To get this behavior, you just say

	__PACKAGE__->has_many( tracks =3D> 'C::Track_Artsit' );

But that's not what we said.

The added bit is the mapping. Since you did not pass a class name to
has_many, but an array ref with a class name and a field name, what you
will actually be getting is not the C::Track_Artsit object, but a sort
of mapping, where each C::Track_Artsit object loaded will have ->track
called on it, and /that/'s what will be returned.

In effect, you do $artist->tracks, and CDBI loads all the lookups, and
then goes to the 'track' of each lookup, and gives you back a number of
C::Track objects.

The reason you get an object is because we previously told
C::Track_Artsit that it has_a C::Track in the column 'track', so the
foreign key is inflated.

1. http://search.cpan.org/~ikebe/Class-DBI-Loader-0.02/lib/Class/DBI/Loader=
=2Epm#SYNOPSIS

--=20
 ()  Yuval Kogman <nothingmuch@xxxxxxxx.xxx> 0xEBD27418  perl hacker &
 /\  kung foo master: /me has realultimatepower.net: neeyah!!!!!!!!!!!!


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