| Maypole::Manual::Model - Maypole Model Classes |
Maypole::Manual::Model - Maypole Model Classes
Maypole's model classes provide an interface to your data store. In principle Maypole can connect to pretty much any data source, but the default model is based on the popular the Class::DBI manpage object interface that uses the near-universal the DBI manpage Perl interface to databases.
Maypole model classes are pretty evil. The very first thing a Maypole model class will do in a Maypole application is to cause a load of table-based classes to come into being, and then assimilate them.
What I mean by this is that when you set up a Maypole application, in your driver class, you'll say something like this:
BeerDB->setup("dbi:mysql:beerdb");
setup is a Maypole method, and it hands its parameter to the model
class. In our case, the argument is a DBI connect string, because
that's what Maypole::Model::CDBI, the Class::DBI-based model
expects. Maypole::Model::CDBI has a method called setup_database
that creates all the Class::DBI table classes after connecting to the
database with that connect string. It does this by using
Class::DBI::Loader, a utility module which asks a database
about its schema and sets up classes such as BeerDB::Beer to inherit
from Class::DBI.
Now it gets interesting. The names of these classes are stashed away in
the application's configuration, and then Maypole forcibly has these
classes inherit from the model class. Our BeerDB::Beer now inherits
both from Class::DBI and Maypole::Model::CDBI.
This is useful for two reasons. The first reason is that
Maypole::Model::CDBI is stuffed full of Class::DBI goodies that
make writing Maypole applications a lot easier:
package Maypole::Model::CDBI;
use base qw(Maypole::Model::Base Class::DBI);
use Maypole::Model::CDBI::AsForm;
use Class::DBI::FromCGI; # probabyly broken .
use Class::DBI::Loader;
use Class::DBI::AbstractSearch;
use Class::DBI::Plugin::RetrieveAll;
use Class::DBI::Pager;
We'll meet most of these goodies in the
Standard Templates and Actions
chapter, where we explain how Maypole::Model::CDBI works.
The second reason why we want our table classes to inherit from
Maypole::Model::CDBI is because it provides a useful set of
default actions. So what's an action, and why are they useful?
Maypole operates primarily by turning URLs into method calls on a model class. All that the model stage of Maypole's operation does, when it comes down to it, is maintain a mapping of tables to classes, and despatch a HTTP request to a method call on the relevant table class. This means that if you request a URL such as
http://localhost/beerdb/brewery/delete/20
Maypole's first stage of operation is to turn that into
BeerDB::Brewery->delete(20). Now, it's slightly more complex than
that. Firstly because it doesn't actually pass the parameter 20, but
it passes an object representing row 20 in the database, but we can
gloss over that for the second. No, the real issue is that Maypole does
not allow you to call just any method in the table class; that would be
somewhat insecure.
Instead, Maypole makes a distinction between the kind of methods that only the class itself and other Perl code can call, and the kind of methods that anyone can call from a URL. This latter set of methods are called exported methods, and exporting is done by means of Perl attributes. You define a method to be exported like so:
sub drink :Exported {
This will allow the user to access /beerdb/beer/drink over the web.
An exported method accompanied with a template to render its output is
sometimes called an action.
Maypole model classes like Maypole::Model::CDBI come with a
relatively handy set of actions which are all you need to set up a CRUD
(Create, Read, Update, Delete)
database front-end: viewing a row in a database, editing it, adding a
new one, deleting, and so on. The most important thing about Maypole,
though, is that it doesn't stop there. You can add your own.
For instance, in our beer database application, we could create a
BeerDB::Beer package, and put some additional actions in there.
package BeerDB::Beer;
sub top_five :Exported {
my ($class, $r) = @_;
$r->objects([ ($r->retrieve_all_sorted_by("score"))[-5..-1] ]);
}
Our action is called as a class method with the Maypole request object.
It uses the Class::DBI::Plugin::RetrieveAll module's
retrieve_all_sorted_by mixin to get the top five scoring beers, and
puts these in the objects slot of the request of object. Next, the
view class will come along and process the top_five template with
these five beers.
We'll look more at how to put together actions in the Standard Templates and Actions chapter and our case studies.
Contents, Next Maypole View Classes, Previous Introduction to Maypole
| Maypole::Manual::Model - Maypole Model Classes |