=head1 The Beer Database, Twice

We briefly introduced the "beer database" example in the L<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?

=head2 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.

=head2 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. 

=head3 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 C<BeerDB>. This package is called the B<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 C<Apache::MVC>,
which is the Apache C<mod_perl> based version of Maypole; there's also
a CGI version, C<CGI::Maypole>, and a command-line version for
debugging, C<Maypole::CLI>, but C<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 L<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, C<Maypole::Model::CDBI>, which takes a
DBI connect string. So this one line declares that we're using a C<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
C<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.

=head3 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 C<config> which returns a hash reference
of the application's whole configuration. We can use this to set some
parameters; the C<uri_base> is used as the canonical URL of the base
of this application, and Maypole uses it to construct links.

By defining C<rows_per_page>, we say that any listings we do with the
C<list> and C<search> templates should be arranged in sets of pages, with
a maximum of 10 items on each page. If we didn't declare that, C<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
C<handpump> which acts as a linking table in a many-to-many relationship
between the C<pub> and C<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 C<display_tables>.

=head3 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 L<StandardTemplates.pod>, this is an set of instructions to
C<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.

=head3 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 C<brewery> table with an ID of 2.

The usual C<Class::DBI> way to do this involves the C<has_a> and
C<has_many> methods, but I can never remember how to use them, so I came
up with the C<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;

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

=head2 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 C<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 C<Maypole::Model::Base>, and
normally, it does a pretty good job; it turns C<model_number> into
"Model Number", for instance, but there was no way it could guess that
C<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 C<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 C<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 C<BeerDB::Beer>
object can call C<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 C<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.
