Just a Theory

By David E. Wheeler

Catalyst with DBIx::Connector and Template::Declare

Following up on my post yesterday introducing Catalyst::View::TD, today I’d like to continue with the next step in chapter 3 of the Catalyst tutorial. The twist here is that I’m going to use PostgreSQL for the database back-end and start introducing some database best practices. I’m also going to make use of my DBIx::Connector module to interact with the database.

Create the Database

Picking up with the database creation section of the tutorial, the first change I’d like to make is to use a natural key for the books table. All books have unique identifiers, known as ISBNs, so creating a surrogate key (the typical id column in ORM-managed applications) is redundant. One of the nice things about PostgreSQL is that it ships with a contributed library, isn, which validates ISBN and other international identifiers. So we use this contrib module (usually included in package-installed PostgreSQL servers) for the primary key for books. If you need to install it from source, it’s pretty easy:

cd postgresql-8.4.1/contrib/isn
make
make install

Ideally I’d use a natural key for the authors table too, but despite some attempts to create universal identifiers for authors, nothing has really caught on as far as I know. So I’ll just stick to a surrogate key for now.

First step: create the database and install isn if it’s not already included in the template database:

createdb -U postgres myapp
psql -U postgres -d myapp -f /usr/local/pgsql/share/contrib/isn.sql

The isn.sql file may be somewhere else on your system. Now let’s create the database. Create sql/001-books.sql in the MyApp directory and paste this into it:

BEGIN;

CREATE TABLE books (
    isbn   ISBN13   PRIMARY KEY,
    title  TEXT     NOT NULL DEFAULT '',
    rating SMALLINT NOT NULL DEFAULT 0 CHECK (rating BETWEEN 0 AND 5)
);

CREATE TABLE authors (
    id         BIGSERIAL PRIMARY KEY,
    surname    TEXT NOT NULL DEFAULT '',
    given_name TEXT NOT NULL DEFAULT ''
);

CREATE TABLE book_author (
    isbn       ISBN13 REFERENCES books(isbn),
    author_id  BIGINT REFERENCES authors(id),
    PRIMARY KEY (isbn, author_id)
);

INSERT INTO books
VALUES ('1587201534',        'CCSP SNRS Exam Certification Guide', 5),
       ('978-0201633467',    'TCP/IP Illustrated, Volume 1',       5),
       ('978-0130183804',    'Internetworking with TCP/IP Vol.1',  4),
       ('978-1-56592-243-3', 'Perl Cookbook',                      5),
       ('978-0735712010',    'Designing with Web Standards',       5)
;

INSERT INTO authors
VALUES (1, 'Greg',      'Bastien'),
       (2, 'Sara',      'Nasseh'),
       (3, 'Christian', 'Degu'),
       (4, 'Richard',   'Stevens'),
       (5, 'Douglas',   'Comer'),
       (6, 'Tom',       'Christiansen'),
       (7, 'Nathan',    'Torkington'),
       (8, 'Jeffrey',   'Zeldman')
;

INSERT INTO book_author
VALUES ('1587201534',        1),
       ('1587201534',        2),
       ('1587201534',        3),
       ('978-0201633467',    4),
       ('978-0130183804',    5),
       ('978-1-56592-243-3', 6),
       ('978-1-56592-243-3', 7),
       ('978-0735712010',    8)
;

COMMIT;

Yeah, I Googled for the ISBNs for those books. I found the ISBN-13 number for most of them, but it handles the old ISBN-10 format, too, automatically upgrading it to ISBN-13. I also added a CHECK constraint for the rating column, to be sure that the value is always BETWEEN 0 AND 5. I also like to include default values where it’s sensible to do so, and that syntax for inserting multiple rows at once is pretty nice to have.

Go ahead and run this against your database:

psql -U postgres -d myapp -f sql/001-books.sql

Now if you connect to the server, you should be able to query things like so:

$ psql -U postgres myapp
psql (8.4.1)
Type "help" for help.

myapp=# select * from books;
       isbn        |               title                | rating 
-------------------+------------------------------------+--------
 978-1-58720-153-0 | CCSP SNRS Exam Certification Guide |      5
 978-0-201-63346-7 | TCP/IP Illustrated, Volume 1       |      5
 978-0-13-018380-4 | Internetworking with TCP/IP Vol.1  |      4
 978-1-56592-243-3 | Perl Cookbook                      |      5
 978-0-7357-1201-0 | Designing with Web Standards       |      5
(5 rows)

Setup the Database Connection

Great! The database is set. Now we need a way for the app to talk to it. I’ve not yet decided how I’m going to integrate DBIx::Connector into a Catalyst model class; maybe I’ll figure it out as I write these posts. But since my mantra is “the database is the model,â€? for now I won’t bother with a model at all. Instead, I’ll create a simple accessor in MyApp so we can easily get at the database connection wherever we need it. To do that, add these lines to lib/MyApp.pm:

use Moose;
use DBIx::Connector;
use Exception::Class::DBI;

has conn => (is => 'ro', lazy => 1, default => sub {
    DBIx::Connector->new( 'dbi:Pg:dbname=myapp', 'postgres', '', {
        PrintError     => 0,
        RaiseError     => 0,
        HandleError    => Exception::Class::DBI->handler,
        AutoCommit     => 1,
        pg_enable_utf8 => 1,
    });
});

We load Moose to get the has keyword, the officially sanctioned interface for defining attributes in Catalyst classes. Then I use that keyword to create the conn attribute. This attribute is read-only and has a DBIx::Connector object for its default value. The nice thing about this is that the DBIx::Connector object won’t be instantiated until it’s actually needed, and then it will be kept forever. We never have to do anything else to use it.

Oh, and I like to make sure that text data coming back from PostgreSQL is properly encoded as UTF-8, and I like to use Exception::Class::DBI to turn DBI errors into exception objects.

Now it’s time to update our controller and template to fetch actual data from the database. Edit lib/MyApp/Controller/Books.pm and change the list method to:

sub list : Local {
    my ($self, $c) = @_;
    $c->stash->{books} = $c->conn->run(fixup => sub {
        my $sth = $_->prepare('SELECT isbn, title, rating FROM books');
        $sth->execute;
        $sth;
    });
}

All we’re doing here is creating a statement handle for the query, executing the query, and storing the statement handle in the stash. Now we need to update the template to use the statement handle. Open up lib/MyApp/Templates/HTML/Books.pm and change the list template to:

template list => sub {
    my ($self, $args) = @_;
    table {
        row {
            th { 'Title'  };
            th { 'Rating' };
            th { 'Author' };
        };
        my $sth = $args->{books};
        while (my $book = $sth->fetchrow_hashref) {
            row {
                cell { $book->{title}  };
                cell { $book->{rating} };
                cell { $book->{author} };
            };
        };
    };
};

All we do is fetch each row from the statement handle and output it. The only thing that’s changed is the use of the statement handle as an iterator rather than an array reference.

And now we’re set! Restart your server with script/myapp_server.pl and point your browser at http://localhost:3000/books/list. Now you should see the book titles and ratings, though the authors still aren’t present. We’ll fix that in a later post.

Takeaway

The takeaway from this post: Use PostgreSQL’s support for custom data types to create validated natural keys for your data, and use a stable, persistent database connection to talk directly to the database. No need for an ORM here, as the DBI provides a very Perlish access to a very capable DSL for models called SQL.

More soon.

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