What is Maypole?

For instance, you may know that Maypole is extremely good at putting web front-ends onto databases. This is true, but it's only a part of what Maypole does. You may have heard that Maypole is a web application framework, which is true, but it doesn't mean very much. There are a huge number of things that Maypole can do, because it's very much a blank slate. You can make it do what you will. In this manual, we'll be making it act as a front-end to a database, as a social network site, as an intranet portal, and many other things besides. It is a framework.

I like to think that Maypole is a way of going from a URL to a method call to some output. If you have a URL like "/product/order/12", Maypole is a way of having it load up product number 12, call an "order" method, and produce a page about what it's just done. The reason Maypole is such a big deal is because it does all this for you. You no longer have to care about your web server. You hardly have to care about your database. You don't have to care about templating modules, parsing CGI parameters, or anything else. You only need to care about business logic, and the business logic in this instance is how you "order" a product, and what you need to display about it once you've done so. This is what programming should be: only caring about the work that distinguishes one program from another.

It does this using a technique called MVC for web applications.

What is MVC for web applications?

This design pattern is found primarily in graphical applications; the idea is that you have a Model class which represents and manipulates your data, a View class which is responsible for displaying that data to the user, and a Controller class which controls the other classes in response to events triggered by the user. This analogy doesn't correspond precisely to a web-based application, but we can take an important principle from it. As Template Toolkit author Andy Wardley explains:

    What the MVC-for-the-web crowd are really trying to achieve is a clear
    separation of concerns.  Put your database code in one place, your 
    application code in another, your presentation code in a third place.  
    That way, you can chop and change different elements at will,
    hopefully without affecting the other parts (depending on how well your
    concerns are separated, of course).  This is common sense and good practice.
    MVC achieves this separation of concerns as a by-product of clearly 
    separating inputs (controls) and outputs (views).

This is what Maypole does. It has a number of database drivers, a number of front-end drivers and a number of templating presentation drivers. In common cases, Maypole provides precisely what you need for all of these areas, and you get to concentrate on writing just the business logic of your application. This is one of the reasons why Maypole lets you develop so rapidly: because most of the time, you don't need to do any development at all.

The Beer Database example

Because Maypole is all about beer. If you look carefully, you can probably see men playing cricket on the village green. The first ever Maypole application was written to help me keep track of the many different ales available in my area - their styles, their tastes, their breweries, prices and so on. Then the more I thought about it, the more I thought it was a particularly good data model for demonstrating different kinds of database relationships.

We have a "brewery" table, which has several "beer"s. We'll call this a has-many relationship. The beers each have a "style"; styles are stored in a separate table, so "beer" has-a "style". Beers are in several pubs and a pub has several beers, so beers and pubs are in a many-to-many relationship. We use a link table called "handpump" to relate pubs to beers.

All in all, this gives us a schema like the following:

    create table brewery (
        id int not null auto_increment primary key,
        name varchar(30),
        url varchar(50),
        notes text
    );

    create table beer (
        id int not null auto_increment primary key,
        brewery integer,
        style integer, 
        name varchar(30),
        url varchar(120),
        score integer(2),
        price varchar(12),
        abv varchar(10),
        notes text
    );

    create table handpump (
        id int not null auto_increment primary key,
        beer integer,
        pub integer
    );

    create table pub (
        id int not null auto_increment primary key,
        name varchar(60),
        url varchar(120),
        notes text
    );

    create table style (
        id int not null auto_increment primary key,
        name varchar(60),
        notes text
    );

If you have "DBD::SQLite" available, then a database like this will be created when Maypole was installed. Let's now see how to set it up with a web interface.

Setting up Maypole

The next thing we need is a module which is going to do all the work. Thankfully, it doesn't need to do all the work itself. It's going to be a subclass of "Maypole" or a Maypole front-end like "Apache::MVC". It roughly corresponds to the controller in an MVC design, and is also referred to as the driver, handler or request.

Here's the driver class for our beer database application. We're not going to go into much detail about it here; we'll do that in the Beer Database chapter. For now, simply admire its brevity, as you realise this is all the code you need to write for a simple database front-end:

    package BeerDB;
    use Maypole::Application;
    BeerDB->setup("dbi:SQLite:t/beerdb.db");
    BeerDB->config->uri_base("http://localhost/beerdb");
    BeerDB->config->template_root("/path/to/templates");
    BeerDB->config->rows_per_page(10);
    BeerDB->config->display_tables([qw[beer brewery pub style]]);
    BeerDB::Brewery->untaint_columns( printable => [qw/name notes url/] );
    BeerDB::Style->untaint_columns( printable => [qw/name notes/] );
    BeerDB::Beer->untaint_columns(
        printable => [qw/abv name price notes/],
        integer => [qw/style brewery score/],
        date => [ qw/date/],
    );

    use Class::DBI::Loader::Relationship;
    BeerDB->config->{loader}->relationship($_) for (
        "a brewery produces beers",
        "a style defines beers",
        "a pub has beers on handpumps");
    1;

There's a version of this program in the examples/ directory in the Maypole files that you downloaded in the ~root/.cpan/ build area. This defines the "BeerDB" application. To set it up as a mod_perl handler, just tell the Apache configuration about it:

    <Location /beerdb>
        SetHandler perl-script
        PerlHandler BeerDB
    </Location>

To use it as a CGI script, put it in your cgi-bin directory, together with a small file called beer.cgi:

    #!/usr/bin/perl
    use strict;
    use warnings;
    use BeerDB;
    BeerDB->run();

and change one line in "BeerDB.pm":

    BeerDB->config->uri_base("http://localhost/cgi-bin/beer.cgi");

And now we need some templates. As we'll see in the chapter on views, there are several types of template. We're going to copy the whole lot from the templates/ directory of the Maypole source package into the /beerdb directory under our web root. Make the "template_root" in "BeerDB" agree with your path.

And that's it. We should now be able to go to "http://localhost/beerdb/" or "http://localhost/cgi-bin/beer.cgi/" and see a menu of things to browse; "http://localhost/beerdb/beer/list" will give a list of beers. There might not be any yet. There's a box that lets you add them.

If you have any problems getting to this point, you might want to look at <http://maypole.perl.org>. There's a FAQ and a link to a mailing list.

Play about with the site. Add some beers. Maybe go out and buy some beers to review if you need some inspiration. Don't be ill on my carpet.

Phases of a Maypole request

Remember that, for instance, we don't ever tell Maypole what tables our database has, or what columns each table has. We don't tell Maypole what those tables should be called or how to display them. We don't tell Maypole what to do - that we want to list, search, edit and delete beers and breweries. Maypole just works that out for itself. We can customize it and have Maypole do all sorts of interesting things with our database, and most of the rest of this manual will be about how to do that.

In order to do that, we need to look at what Maypole's actually doing. Here's a quick overview, there's more detail in the Workflow chapter.

As mentioned, Maypole is responsible for turning a URL into an object, a method call, and some templated output.

Maypole's process revolves around the concept of the Maypole request object. This is a little like Apache's request object, but at a much higher level - in fact, in "mod_perl"-based Maypole front-ends, the Apache request object is incorporated in the Maypole request object. All that Maypole does is gradually flesh out this object until it contains something in the "output" member, and then it is dispatched back to the front-end for output.

So to start with, we take the Apache request (or CGI object, or other way of isolating what's going on) and break it down. For instance, we turn the URL "/beer/view/1" into

    {
        table => "beer",
        action => "view",
        args => [ 1 ]
    }

Then Maypole will check that "beer" is a real table, and find the class that models it. It also checks whether or not we're allowed to call the "view" method over the network:

    {
        table => "beer",
        action => "view",
        args => [ 1 ],
        model_class => "BeerDB::Beer"
    }

Then there's a user-defined authentication method, which by default just lets us do anything. Now we hand over to the model class, which loads up the object, and decides what template we want to use:

    {
        table => "beer",
        action => "view",
        args => [ ],
        objects => [ BeerDB::Beer->retrieve(1) ],
        model_class => "BeerDB::Beer",
        template => "view"
    }

Then it calls "BeerDB::Beer->view", passing in the request object as a parameter, and passes the whole lot to the view class for templating. In the next two chapters, we'll look at how Maypole's default model and view classes generally do what you want them to do.

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