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DBIx-Class-ResultSource-MultipleTableInheritance-0.03
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/ DBIx::Class::ResultSource::MultipleTableInheritance

NAME

DBIx::Class::ResultSource::MultipleTableInheritance Use multiple tables to define your classes

NOTICE

This only works with PostgreSQL at the moment. It has been tested with PostgreSQL 9.0, 9.1 beta, and 9.1.

There is one additional caveat: the "parent" result classes that you defined with this resultsource must have one primary column and it must be named "id."

SYNOPSIS

{
    package Cafe::Result::Coffee;

    use strict;
    use warnings;
    use parent 'DBIx::Class::Core';
    use aliased 'DBIx::Class::ResultSource::MultipleTableInheritance'
        => 'MTI';

    __PACKAGE__->table_class(MTI);
    __PACKAGE__->table('coffee');
    __PACKAGE__->add_columns(
        "id", { data_type => "integer" },
        "flavor", {
            data_type => "text",
            default_value => "good" },
    );

    __PACKAGE__->set_primary_key("id");

    1;
}

{
    package Cafe::Result::Sumatra;

    use parent 'Cafe::Result::Coffee';

    __PACKAGE__->table('sumatra');

    __PACKAGE__->add_columns( "aroma",
        { data_type => "text" }
    );

    1;
}

...

my $schema = Cafe->connect($dsn,$user,$pass);

my $cup = $schema->resultset('Sumatra');

print STDERR Dwarn $cup->result_source->columns;

    "id"
    "flavor"
    "aroma"
    ..

Inherit from this package and you can make a resultset class from a view, but that's more than a little bit misleading: the result is transparently writable.

This is accomplished through the use of stored procedures that map changes written to the view to changes to the underlying concrete tables.

WHY?

In many applications, many classes are subclasses of others. Let's say you have this schema:

# Conceptual domain model

class User {
    has id,
    has name,
    has password
}

class Investor {
    has id,
    has name,
    has password,
    has dollars
}

That's redundant. Hold on a sec...

class User {
    has id,
    has name,
    has password
}

class Investor extends User {
    has dollars
}

Good idea, but how to put this into code?

One far-too common and absolutely horrendous solution is to have a "checkbox" in your database: a nullable "investor" column, which entails a nullable "dollars" column, in the user table.

create table "user" (
    "id" integer not null primary key autoincrement,
    "name" text not null,
    "password" text not null,
    "investor" tinyint(1),
    "dollars" integer
);

Let's not discuss that further.

A second, better, solution is to break out the two tables into user and investor:

create table "user" (
    "id" integer not null primary key autoincrement,
    "name" text not null,
    "password" text not null
);

create table "investor" (
    "id" integer not null references user("id"),
    "dollars" integer
);

So that investor's PK is just an FK to the user. We can clearly see the class hierarchy here, in which investor is a subclass of user. In DBIx::Class applications, this second strategy looks like:

my $user_rs = $schema->resultset('User');
my $new_user = $user_rs->create(
    name => $args->{name},
    password => $args->{password},
);

...

my $new_investor = $schema->resultset('Investor')->create(
    id => $new_user->id,
    dollars => $args->{dollars},
);

One can cope well with the second strategy, and it seems to be the most popular smart choice.

HOW?

There is a third strategy implemented here. Make the database do more of the work: hide the nasty bits so we don't have to handle them unless we really want to. It'll save us some typing and it'll make for more expressive code. What if we could do this:

my $new_investor = $schema->resultset('Investor')->create(
    name => $args->{name},
    password => $args->{password},
    dollars => $args->{dollars},
);

And have it Just Work? The user...

{
    name => $args->{name},
    password => $args->{password},
}

should be created behind the scenes, and the use of either user or investor in your code should require no special handling. Deleting and updating $new_investor should also delete or update the user row.

It does. User and investor are both views, their concrete tables abstracted away behind a set of rules and triggers. You would expect the above DBIC create statement to look like this in SQL:

INSERT INTO investor ("name","password","dollars") VALUES (...);

But using MTI, it is really this:

INSERT INTO _user_table ("username","password") VALUES (...);
INSERT INTO _investor_table ("id","dollars") VALUES (currval('_user_table_id_seq',...) );

For deletes, the triggers fire in reverse, to preserve referential integrity (foreign key constraints). For instance:

my $investor = $schema->resultset('Investor')->find({id => $args->{id}});
$investor->delete;

Becomes:

DELETE FROM _investor_table WHERE ("id" = ?);
DELETE FROM _user_table WHERE ("id" = ?);

METHODS

new

MTI find the parents, if any, of your resultset class and adds them to the list of parent_sources for the table.

add_additional_parents

Continuing with coffee:

__PACKAGE__->result_source_instance->add_additional_parents(
    qw/
        MyApp::Schema::Result::Beverage
        MyApp::Schema::Result::Liquid
    /
);

This just lets you manually add additional parents beyond the ones MTI finds.

add_additional_parent
__PACKAGE__->result_source_instance->add_additional_parent(
        MyApp::Schema::Result::Beverage
);

You can also add just one.

attach_additional_sources

MTI takes the parents' sources and relationships, creates a new DBIx::Class::Table object from them, and registers this as a new, raw, source in the schema, e.g.,

use MyApp::Schema;

print STDERR map { "$_\n" } MyApp::Schema->sources;

# Coffee
# Beverage
# Liquid
# Sumatra
# Raw::Sumatra

Raw::Sumatra will be used to generate the view.

view_definition

This takes the raw table and generates the view (and stored procedures) you will use.

AUTHOR

Matt S. Trout, <mst@shadowcatsystems.co.uk>

CONTRIBUTORS

Amiri Barksdale, <amiri@roosterpirates.com>

COPYRIGHT

Copyright (c) 2011 the DBIx::Class::ResultSource::MultipleTableInheritance "AUTHOR" and "CONTRIBUTORS" as listed above.

LICENSE

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO

DBIx::Class DBIx::Class::ResultSource