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Maypole-1.99_01
River stage two • 11 direct dependents • 13 total dependents
/ Maypole::Manual::Beer

The Beer Database, Twice

We briefly introduced the "beer database" example in the About.pod material, where we presented its driver class as a fait accompli. Where did all that code come from, and what does it actually mean?

The big beer problem

I have a seriously bad habit. This is not the beer problem; this is a programming problem. The bad habit is that when I approach a problem I want to solve, I get sidetracked deeper and deeper trying to solve more and more generic problems, and then, satisfied with solving the generic problem, I never get around to solving the specific problem. I always write libraries for people writing libraries, and never write applications.

The thing with really good beer is that it commands you to drink more of it, and then by the morning you can't remember whether it was any good or not. After buying several bottles of some random central African lager on a dim recollection that it was really good and having it turn out to be abysmal, this really became a problem. If only I could have a database that I updated every time I buy a new beer, I'd be able to tell whether or not I should buy that Lithuanian porter again or whether it would be quicker just to flush my money down the toilet and cut out the middle-man.

The only problem with databases on Unix is that there isn't really a nice way to get data into them. There isn't really a Microsoft Access equivalent which can put a simple forms-based front-end onto an arbitrary database, and if there is, I either didn't like it or couldn't find it, and after a few brews, you really don't want to be trying to type in your tasting notes in raw SQL.

So you see a generic problem arising out of a specific problem here. I didn't want to solve the specific problem of the beer database, because I'd already had another idea for a database that needed a front-end. So for two years, I sat on this great idea of having a database of tasting notes for beer. I even bought that damned African beer again. Enough was enough. I wrote Maypole.

The easy way

The first Maypole application was the beer database. We've already met it; it looks like this.

package BeerDB;
use base 'Apache::MVC';
BeerDB->set_database("dbi:SQLite:t/beerdb.db");
BeerDB->config->{uri_base} = "http://localhost/beerdb/";
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;

Now, we can split this into four sections. Let's look at them one at a time.

Driver setup

Here's the first section:

package BeerDB;
use base 'Apache::MVC';
BeerDB->setup("dbi:SQLite:t/beerdb.db");

This is actually all you need for a functional database front-end. Everything else is configuration details. This says three things: we're an application called BeerDB. This package is called the driver class, because it's a relatively small class which defines how the whole application is going to run.

The second line says that our front-end is going to be Apache::MVC, which is the Apache mod_perl based version of Maypole; there's also a CGI version, CGI::Maypole, and a command-line version for debugging, Maypole::CLI, but Apache::MVC is usually the one you want.

Thirdly we're going to need to set up our database with the given DBI connection string. Now the core of Maypole itself doesn't know about DBI; as we explained in Model.pod, this argument is passed to our model class wholesale. As we haven't said anything about a model class, we get the default one, Maypole::Model::CDBI, which takes a DBI connect string. So this one line declares that we're using a CDBI model class and it sets up the database for us. In the same way, we don't say that we want a particular view class, so we get the default Maypole::View::TT.

At this point, everything is in place; we have our driver class, it uses a front-end, we have a model class and a view class, and we have a data source.

Application configuration

The next of our four sections is the configuration for the application itself.

BeerDB->config->{uri_base} = "http://localhost/beerdb/";
BeerDB->config->{rows_per_page} = 10;
BeerDB->config->{display_tables} = [qw[beer brewery pub style]];

Maypole provides a method called config which returns a hash reference of the application's whole configuration. We can use this to set some parameters; the uri_base is used as the canonical URL of the base of this application, and Maypole uses it to construct links.

By defining rows_per_page, we say that any listings we do with the list and search templates should be arranged in sets of pages, with a maximum of 10 items on each page. If we didn't declare that, list would try to put all the objects on one page, which could well be bad.

Finally, we declare which tables we want our Maypole front-end to reference. If you remember from the schema, there's a table called handpump which acts as a linking table in a many-to-many relationship between the pub and beer tables. As it's only a linking table, we don't want people poking with it directly, so we exclude it from the list of display_tables.

Editability

The next section is the following set of lines:

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/],
);

As explained in StandardTemplates.pod, this is an set of instructions to Class::DBI::FromCGI regarding how the given columns should be edited. If we didn't have this section, we'd be able to view and delete records, but adding and editing them wouldn't work. It took me ages to work that one out.

Relationships

Finally, we want to explain to Maypole how the various tables relate to each other. This is done so that, for instance, when displaying a beer, the brewery does not appear as an integer like "2" but as the name of the brewery from the brewery table with an ID of 2.

The usual Class::DBI way to do this involves the has_a and has_many methods, but I can never remember how to use them, so I came up with the Class::DBI::Loader::Relationship module; this was another yak that needed shaving on the way to the beer database:

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;

CDBIL::Relationship acts on a Class::DBI::Loader object and defines relationships between tables in a fairly free-form style. The equivalent in ordinary Class::DBI would be:

BeerDB::Brewery->has_many(beers => "BeerDB::Beer");
BeerDB::Beer->has_a(brewery => "BeerDB::Brewery");
BeerDB::Style->has_many(beers => "BeerDB::Beer");
BeerDB::Beer->has_a(style => "BeerDB::Style");

BeerDB::Handpump->has_a(beer => "BeerDB::Beer");
BeerDB::Handpump->has_a(pub => "BeerDB::Pub");
BeerDB::Pub->has_many(beers => [ BeerDB::Handpump => 'beer' ]);
BeerDB::Beer->has_many(pubs => [ BeerDB::Handpump => 'pub' ]);

Maypole's default templates will use this information to display, for instance, a list of a brewery's beers on the brewery view page.

This is the complete beer database application; Maypole's default templates and the actions in the view class do the rest. But what if we want to do a little more. How would we begin to extend this application?

The hard way

Maypole was written because I don't like writing more Perl code than is necessary. I also don't like writing HTML. In fact, I don't really get on this whole computer thing, to be honest. But we'll start by ways that we can customize the beer application simply by adding methods or changing properties of the Perl driver code.

The first thing we ought to look into is the names of the columns; most of them are fine, but that "Abv" column stands out. I'd rather that was "A.B.V.". Maypole uses the column_names method to map between the names of the columns in the database to the names it displays in the default templates. This is provided by Maypole::Model::Base, and normally, it does a pretty good job; it turns model_number into "Model Number", for instance, but there was no way it could guess that abv was an abbreviation. Since it returns a hash, the easiest way to correct it is to construct a hash consisting of the bits it got right, and then override the bits it got wrong:

package BeerDB::Beer;
sub column_names { 
    (shift->SUPER::column_names(), abv => "A.B.V.")
}

Similarly, the order of columns is a bit wonky. We can fix this by overriding the display_columns method; this is also a good way to hide away any columns we don't want to have displayed, in the same way as declaring the display_tables configuration parameter let us hide away tables we weren't using:

sub display_columns { 
    ("name", "brewery", "style", "price", "score", "abv", "notes")
}

Hey, have you noticed that we haven't done anything with the beers/handpumps/pubs thing yet? Good, I was hoping that you hadn't. Ayway, this is because Maypole can't tell easily that a BeerDB::Beer object can call pubs to get a list of pubs. Not yet, at least; we're working on it. In the interim, we can explicitly tell Maypole which accessors are related to the BeerDB::Beer class like so:

sub related { "pubs" }

Now when we view a beer, we'll have a list of the pubs that it's on at.