---
title: A simple feed aggregator with modern Perl - part 1
category: perl
layout: post
---

Following "matt's post":http://www.shadowcat.co.uk/blog/matt-s-trout/iron-man/ about people not blogging enough about Perl, I've decided to try to post once a week about Perl. So I will start by a series of articles about what we call *modern Perl*. For this, I will write a simple feed agregator (using "Moose":http://search.cpan.org/~drolsky/Moose-0.75/lib/Moose.pm, "DBIx::Class":http://search.cpan.org/perldoc?DBIx::Class, "KiokuDB":http://search.cpan.org/perldoc?KiokuDB, some tests, and a basic frontend (with "Catalyst":http://search.cpan.org/perldoc?Catalyst). This article will be split in four parts:

 * the first one will explain how to create a schema using *DBIx::Class*
 * the second will be about the aggregator. I will use *Moose** and **KiokuDB*
 * the third one will be about writing tests with *Test::Class*
 * the last one will focus on *Catalyst*

The code of these modules will be available on my github account at the
same time each article is published.

bc. disclaimer:
I'm not showing you how to write the perfect feed aggregator.  The purpose of
this series of articles is only to show you how to write a simple aggregator
using modern Perl.


h3. The database schema

We will use a database to store a list of feeds and feed entries. As I don't like, no, wait, I *hate* SQL, I will use an ORM for accessing the database. For this, my choice is *DBIx::Class*, the best ORM available in Perl.

bc. If you never have used an ORM before, ORM stands for Object Relational
Mapping. It's a SQL to OO mapper that creates an abstract encapsulation of
your databases operations. *DBIx::Class*' purpose is to represent "queries in
your code as perl-ish as possible.

For a basic aggregator we need:

 * a table for the list of feeds
 * a table for the entries

We will create these two tables using *DBIx::Class*. For this, we first create a Schema module. I use *Module::Setup*, but you can use *Module::Starter* or whatever you want.

{% highlight bash %}
module-setup MyModel
cd MyModel
vim lib/MyModel.pm
{% endhighlight %}

{% highlight perl %}
package MyModel;
use base qw/DBIx::Class::Schema/;
__PACKAGE__->load_classes();
1;
{% endhighlight %}

So, we have just created a schema class. The *load_classes* method loads all the classes that reside under the *MyModel* namespace. We now create the result class *MyModel::Feed* in *lib/MyModel/Feed.pm*:

{% highlight perl %}
package MyModel::Feed;
use base qw/DBIx::Class/;
__PACKAGE__->load_components(qw/Core/);
__PACKAGE__->table('feed');
__PACKAGE__->add_columns(qw/ feedid url /);
__PACKAGE__->set_primary_key('feedid');
__PACKAGE__->has_many(entries => 'MyModel::Entry', 'feedid');
1;
{% endhighlight %}

Pretty self explanatory: we declare a result class that uses the table feed, with two columns: *feedid* and *url*, *feedid* being the primary key. The *has_many* method declares a one-to-many relationship.

Now the result class *MyModel::Entry* in *lib/MyModel/Entry.pm*:

{% highlight perl %}
package MyModel::Entry;
use base qw/DBIx::Class/;
__PACKAGE__->load_components(qw/Core/);
__PACKAGE__->table('entry');
__PACKAGE__->add_columns(qw/ entryid permalink feedid/);
__PACKAGE__->set_primary_key('entryid');
__PACKAGE__->belongs_to(feed => 'MyModel::Feed', 'feedid');
1;
{% endhighlight %}

Here we declare *feed* as a foreign key, using the column name *feedid*.

You can do a more complex declaration of your schema. Let's say you want to declare the type of your fields, you can do this:

{% highlight perl %}
__PACKAGE__->add_columns(
    'permalink' => {
        'data_type'         => 'TEXT',
        'is_auto_increment' => 0,
        'default_value'     => undef,
        'is_foreign_key'    => 0,
        'name'              => 'url',
        'is_nullable'       => 1,
        'size'              => '65535'
    },
);
{% endhighlight %}

*DBIx::Class* also provides hooks for the deploy command. If you are using MySQL, you may need a InnoDB table. In your class, you can add this:

{% highlight perl %}
sub sqlt_deploy_hook {
    my ($self, $sqlt_table) = @_;
    $sqlt_table->extra(
        mysql_table_type => 'InnoDB',
        mysql_charset    => 'utf8'
    );
}
{% endhighlight %}

next time you call deploy on this table, the hook will be sent to *SQL::Translator::Schema*, and force the type of your table to InnoDB, and the charset to utf8.

Now that we have a *DBIx::Class* schema, we need to deploy it. For this, I always do the same thing: create a *bin/deploy_mymodel.pl* script with the following code:

{% highlight perl %}
use strict;
use feature 'say';
use Getopt::Long;
use lib('lib');
use MyModel;

GetOptions(
    'dsn=s'    => \my $dsn,
    'user=s'   => \my $user,
    'passwd=s' => \my $passwd
) or die usage();

my $schema = MyModel->connect($dsn, $user, $passwd);
say 'deploying schema ...';
$schema->deploy;

say 'done';

sub usage {
    say
        'usage: deploy_mymodel.pl --dsn $dsn --user $user --passwd $passwd';
}
{% endhighlight %}

This script will deploy for you the schema (you need to create the database first if using with mysql).

Executing the following command:

{% highlight bash %}
perl bin/deploy_mymodel.pl --dsn dbi:SQLite:model.db
{% endhighlight %}

generate a *model.db* database so we can work and test it. Now that we got our (really) simple *MyModel* schema, we can start to hack on our aggregator.

"link to the code":http://github.com/franckcuny/ironman-mymodel/tree/master

bc. while using *DBIx::Class*, you may want to take a look at the generated
queries. For this, export *DBIC_TRACE=1* in your environment, and
the queries will be printed on STDERR.