3 Maypole::Manual::Beer - The Beer Database, Twice
7 We briefly introduced the "beer database" example in the
8 L<Introduction to Maypole|Maypole::Manual::About> chapter, where we
9 presented its driver class, C<BeerDB.pm>, as a fait accompli. Where
10 did all that code come from, and what does it actually mean?
12 =head2 The big beer problem
14 I have a seriously bad habit. This is not the beer problem; this is a
15 programming problem. The bad habit is that when I approach a problem I
16 want to solve, I get sidetracked deeper and deeper trying to solve more
17 and more generic problems, and then, satisfied with solving the generic
18 problem, I never get around to solving the specific problem. I always
19 write libraries for people writing libraries, and never write
22 The thing with really good beer is that it commands you to drink more of
23 it, and then by the morning you can't remember whether it was any good
24 or not. After buying several bottles of some random central African
25 lager on a dim recollection that it was really good and having it turn
26 out to be abysmal, this really became a problem. If only I could have a
27 database that I updated every time I buy a new beer, I'd be able to tell
28 whether or not I should buy that Lithuanian porter again or whether it
29 would be quicker just to flush my money down the toilet and cut out the
32 The only problem with databases on Unix is that there isn't really a
33 nice way to get data into them. There isn't really a Microsoft Access
34 equivalent which can put a simple forms-based front-end onto an
35 arbitrary database, and if there is, I either didn't like it or couldn't
36 find it, and after a few brews, you really don't want to be trying to type
37 in your tasting notes in raw SQL.
39 So you see a generic problem arising out of a specific problem here. I
40 didn't want to solve the specific problem of the beer database, because
41 I'd already had another idea for a database that needed a front-end. So
42 for two years, I sat on this great idea of having a database of tasting
43 notes for beer. I even bought that damned African beer again. Enough was
44 enough. I wrote Maypole.
48 The first Maypole application was the beer database. We've already met
49 it; it looks like this.
52 use Maypole::Application;
53 BeerDB->setup("dbi:SQLite:t/beerdb.db");
54 BeerDB->config->uri_base("http://localhost/beerdb");
55 BeerDB->config->template_root("/path/to/templates");
56 BeerDB->config->rows_per_page(10);
57 BeerDB->config->display_tables([qw[beer brewery pub style]]);
58 BeerDB::Brewery->untaint_columns( printable => [qw/name notes url/] );
59 BeerDB::Style->untaint_columns( printable => [qw/name notes/] );
60 BeerDB::Beer->untaint_columns(
61 printable => [qw/abv name price notes/],
62 integer => [qw/style brewery score/],
66 use Class::DBI::Loader::Relationship;
67 BeerDB->config->{loader}->relationship($_) for (
68 "a brewery produces beers",
69 "a style defines beers",
70 "a pub has beers on handpumps");
73 Now, we can split this into four sections. Let's look at them one
78 Here's the first section:
81 use Maypole::Application;
82 BeerDB->setup("dbi:SQLite:t/beerdb.db");
84 This is actually all you need for a functional database front-end. Everything
85 else is configuration details. This says three things: we're an application
86 called C<BeerDB>. This package is called the B<driver class>, because
87 it's a relatively small class which defines how the whole application is
90 The second line says that our front-end is going to be
91 L<Maypole::Application>, it automatically detects if you're using
92 mod_perl or CGI and loads everything necessary for you.
94 Thirdly we're going to need to set up our database with the given DBI
95 connection string. Now the core of Maypole itself doesn't know about
96 DBI; as we explained in the L<Model|Maypole::Manual::Model> chapter,
97 this argument is passed to our
98 model class wholesale. As we haven't said anything about a model
99 class, we get the default one, L<Maypole::Model::CDBI>, which takes a
100 DBI connect string. So this one line declares that we're using a C<CDBI>
101 model class and it sets up the database for us. In the same way, we
102 don't say that we want a particular view class, so we get the default
103 L<Maypole::View::TT>.
105 At this point, everything is in place; we have our driver class, it uses
106 a front-end, we have a model class and a view class, and we have a data
109 =head3 Application configuration
111 The next of our four sections is the configuration for the application itself.
113 BeerDB->config->uri_base("http://localhost/beerdb");
114 BeerDB->config->template_root("/path/to/templates");
115 BeerDB->config->rows_per_page(10);
116 BeerDB->config->display_tables([qw[beer brewery pub style]]);
118 Maypole provides a method called C<config> which returns an object that
119 holds the application's whole configuration. We can use this to set some
120 parameters; the C<uri_base> is used as the canonical URL of the base
121 of this application, and Maypole uses it to construct links.
123 We also tell Maypole where we keep our template files, using
126 By defining C<rows_per_page>, we say that any listings we do with the
127 C<list> and C<search> templates should be arranged in sets of pages, with
128 a maximum of 10 items on each page. If we didn't declare that, C<list>
129 would try to put all the objects on one page, which could well be bad.
131 Finally, we declare which tables we want our Maypole front-end to
132 reference. If you remember from the schema, there's a table called
133 C<handpump> which acts as a linking table in a many-to-many relationship
134 between the C<pub> and C<beer> tables. As it's only a linking table, we
135 don't want people poking with it directly, so we exclude it from the
136 list of C<display_tables>.
140 The next section is the following set of lines:
142 BeerDB::Brewery->untaint_columns( printable => [qw/name notes url/] );
143 BeerDB::Style->untaint_columns( printable => [qw/name notes/] );
144 BeerDB::Beer->untaint_columns(
145 printable => [qw/abv name price notes/],
146 integer => [qw/style brewery score/],
151 L<Standard Templates|Maypole::Manual::StandardTemplates> chapter,
152 this is an set of instructions to
153 L<Class::DBI::FromCGI> regarding how the given columns should be edited.
154 If we didn't have this section, we'd be able to view and delete records,
155 but adding and editing them wouldn't work. It took me ages to work that
160 Finally, we want to explain to Maypole how the various tables relate to
161 each other. This is done so that, for instance, when displaying a beer,
162 the brewery does not appear as an integer like "2" but as the name of
163 the brewery from the C<brewery> table with an ID of 2.
165 The usual L<Class::DBI> way to do this involves the C<has_a> and
166 C<has_many> methods, but I can never remember how to use them, so I came
167 up with the L<Class::DBI::Loader::Relationship> module; this was another
168 yak that needed shaving on the way to the beer database:
170 use Class::DBI::Loader::Relationship;
171 BeerDB->config->{loader}->relationship($_) for (
172 "a brewery produces beers",
173 "a style defines beers",
174 "a pub has beers on handpumps");
177 C<CDBIL::Relationship> acts on a L<Class::DBI::Loader> object and
178 defines relationships between tables in a fairly free-form style.
179 The equivalent in ordinary C<Class::DBI> would be:
181 BeerDB::Brewery->has_many(beers => "BeerDB::Beer");
182 BeerDB::Beer->has_a(brewery => "BeerDB::Brewery");
183 BeerDB::Style->has_many(beers => "BeerDB::Beer");
184 BeerDB::Beer->has_a(style => "BeerDB::Style");
186 BeerDB::Handpump->has_a(beer => "BeerDB::Beer");
187 BeerDB::Handpump->has_a(pub => "BeerDB::Pub");
188 BeerDB::Pub->has_many(beers => [ BeerDB::Handpump => 'beer' ]);
189 BeerDB::Beer->has_many(pubs => [ BeerDB::Handpump => 'pub' ]);
191 Maypole's default templates will use this information to display, for
192 instance, a list of a brewery's beers on the brewery view page.
194 This is the complete beer database application; Maypole's default templates
195 and the actions in the view class do the rest. But what if we want to do a
196 little more. How would we begin to extend this application?
200 Maypole was written because I don't like writing more Perl code than is
201 necessary. I also don't like writing HTML. In fact, I don't really get
202 on this whole computer thing, to be honest. But we'll start by ways that
203 we can customize the beer application simply by adding methods or
204 changing properties of the Perl driver code.
206 The first thing we ought to look into is the names of the columns; most
207 of them are fine, but that "Abv" column stands out. I'd rather that was
208 "A.B.V.". Maypole uses the C<column_names> method to map between the
209 names of the columns in the database to the names it displays in the
210 default templates. This is provided by L<Maypole::Model::Base>, and
211 normally, it does a pretty good job; it turns C<model_number> into
212 "Model Number", for instance, but there was no way it could guess that
213 C<abv> was an abbreviation. Since it returns a hash, the easiest way
214 to correct it is to construct a hash consisting of the bits it got
215 right, and then override the bits it got wrong:
217 package BeerDB::Beer;
219 (shift->SUPER::column_names(), abv => "A.B.V.")
222 There's something to be aware of here: where are you going to type that
223 code? You can just put it in F<BeerDB.pm>. Perl will be happy with that,
224 though you might want to put an extra pair of braces around it to limit
225 the scope of that package declaration. Alternatively, you might think
226 it's neater to put it in a file called F<BeerDB/Beer.pm>, which is the
227 natural home for the package. This would certainly be a good idea if you
228 have a lot of other code to add to the C<BeerDB::Beer> package. But if
229 you do that, you will have to tell Perl to load the F<BeerDB/Beer.pm>
230 file by adding a line to F<BeerDB.pm>:
232 BeerDB::Beer->require;
234 For another example of customization, the order of columns is a bit
235 wonky. We can fix this by
236 overriding the C<display_columns> method; this is also a good way to
237 hide away any columns we don't want to have displayed, in the same way
238 as declaring the C<display_tables> configuration parameter let us hide
239 away tables we weren't using:
241 sub display_columns {
242 ("name", "brewery", "style", "price", "score", "abv", "notes")
245 Hey, have you noticed that we haven't done anything with the
246 beers/handpumps/pubs thing yet? Good, I was hoping that you hadn't.
247 Anyway, this is because Maypole can't tell easily that a C<BeerDB::Beer>
248 object can call C<pubs> to get a list of pubs. Not yet, at least; we're
249 working on it. In the interim, we can explicitly tell Maypole which
250 accessors are related to the C<BeerDB::Beer> class like so:
252 sub related { "pubs" }
254 Now when we view a beer, we'll have a list of the pubs that it's on at.
258 L<Contents|Maypole::Manual>,
259 Next L<The Request Cookbook|Maypole::Manual::Request>,
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