1Maypole::Manual::Beer(3U)ser Contributed Perl DocumentatiMoanypole::Manual::Beer(3)
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6 Maypole::Manual::Beer - The Beer Database, Twice
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9 We briefly introduced the "beer database" example in the Introduction
10 to Maypole chapter, where we presented its driver class, "BeerDB.pm",
11 as a fait accompli. Where did all that code come from, and what does it
12 actually mean?
13 The big beer problem
15 I have a seriously bad habit. This is not the beer problem; this is a
16 programming problem. The bad habit is that when I approach a problem I
17 want to solve, I get sidetracked deeper and deeper trying to solve more
18 and more generic problems, and then, satisfied with solving the generic
19 problem, I never get around to solving the specific problem. I always
20 write libraries for people writing libraries, and never write
21 applications.
22 The thing with really good beer is that it commands you to drink more
24 of it, and then by the morning you can't remember whether it was any
25 good or not. After buying several bottles of some random central
26 African lager on a dim recollection that it was really good and having
27 it turn out to be abysmal, this really became a problem. If only I
28 could have a database that I updated every time I buy a new beer, I'd
29 be able to tell whether or not I should buy that Lithuanian porter
30 again or whether it would be quicker just to flush my money down the
31 toilet and cut out the middle-man.
32 The only problem with databases on Unix is that there isn't really a
34 nice way to get data into them. There isn't really a Microsoft Access
35 equivalent which can put a simple forms-based front-end onto an
36 arbitrary database, and if there is, I either didn't like it or
37 couldn't find it, and after a few brews, you really don't want to be
38 trying to type in your tasting notes in raw SQL.
39 So you see a generic problem arising out of a specific problem here. I
41 didn't want to solve the specific problem of the beer database, because
42 I'd already had another idea for a database that needed a front-end. So
43 for two years, I sat on this great idea of having a database of tasting
44 notes for beer. I even bought that damned African beer again. Enough
45 was enough. I wrote Maypole.
46 The easy way
48 The first Maypole application was the beer database. We've already met
49 it; it looks like this.
50 package BeerDB;
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/],
63 date => [ qw/date/],
64 );
65 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");
71 1;
72 Now, we can split this into four sections. Let's look at them one at a
74 time.
75 Driver setup
77 Here's the first section:
79 package BeerDB;
81 use Maypole::Application;
82 BeerDB->setup("dbi:SQLite:t/beerdb.db");
83 This is actually all you need for a functional database front-end.
85 Everything else is configuration details. This says three things: we're
86 an application called "BeerDB". This package is called the driver
87 class, because it's a relatively small class which defines how the
88 whole application is going to run.
89 The second line says that our front-end is going to be
91 Maypole::Application, it automatically detects if you're using mod_perl
92 or CGI and loads everything necessary for you.
93 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 Model chapter, this argument is passed to
97 our model class wholesale. As we haven't said anything about a model
98 class, we get the default one, Maypole::Model::CDBI, which takes a DBI
99 connect string. So this one line declares that we're using a "CDBI"
100 model class and it sets up the database for us. In the same way, we
101 don't say that we want a particular view class, so we get the default
102 Maypole::View::TT.
103 At this point, everything is in place; we have our driver class, it
105 uses a front-end, we have a model class and a view class, and we have a
106 data source.
107 Application configuration
109 The next of our four sections is the configuration for the application
111 itself.
112 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]]);
117 Maypole provides a method called "config" which returns an object that
119 holds the application's whole configuration. We can use this to set
120 some parameters; the "uri_base" is used as the canonical URL of the
121 base of this application, and Maypole uses it to construct links.
122 We also tell Maypole where we keep our template files, using
124 "template_root".
125 By defining "rows_per_page", we say that any listings we do with the
127 "list" and "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, "list"
129 would try to put all the objects on one page, which could well be bad.
130 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 "handpump" which acts as a linking table in a many-to-many relationship
134 between the "pub" and "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 "display_tables".
137 Editability
139 The next section is the following set of lines:
141 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/],
147 date => [ qw/date/],
148 );
149 As explained in the Standard Templates chapter, this is an set of
151 instructions to Class::DBI::FromCGI regarding how the given columns
152 should be edited. If we didn't have this section, we'd be able to view
153 and delete records, but adding and editing them wouldn't work. It took
154 me ages to work that one out.
155 Relationships
157 Finally, we want to explain to Maypole how the various tables relate to
159 each other. This is done so that, for instance, when displaying a beer,
160 the brewery does not appear as an integer like "2" but as the name of
161 the brewery from the "brewery" table with an ID of 2.
162 The usual Class::DBI way to do this involves the "has_a" and "has_many"
164 methods, but I can never remember how to use them, so I came up with
165 the Class::DBI::Loader::Relationship module; this was another yak that
166 needed shaving on the way to the beer database:
167 use Class::DBI::Loader::Relationship;
169 BeerDB->config->{loader}->relationship($_) for (
170 "a brewery produces beers",
171 "a style defines beers",
172 "a pub has beers on handpumps");
173 1;
174 "CDBIL::Relationship" acts on a Class::DBI::Loader object and defines
176 relationships between tables in a fairly free-form style. The
177 equivalent in ordinary "Class::DBI" would be:
178 BeerDB::Brewery->has_many(beers => "BeerDB::Beer");
180 BeerDB::Beer->has_a(brewery => "BeerDB::Brewery");
181 BeerDB::Style->has_many(beers => "BeerDB::Beer");
182 BeerDB::Beer->has_a(style => "BeerDB::Style");
183 BeerDB::Handpump->has_a(beer => "BeerDB::Beer");
185 BeerDB::Handpump->has_a(pub => "BeerDB::Pub");
186 BeerDB::Pub->has_many(beers => [ 'BeerDB::Handpump' => 'beer' ]);
187 BeerDB::Beer->has_many(pubs => [ 'BeerDB::Handpump' => 'pub' ]);
188 Maypole's default templates will use this information to display, for
190 instance, a list of a brewery's beers on the brewery view page.
191 Note the quoting in 'BeerDB::Handpump' => 'beer', if you forget to
193 quote the left side when using strict you will get compilation errors.
194 This is the complete beer database application; Maypole's default
196 templates and the actions in the view class do the rest. But what if we
197 want to do a little more. How would we begin to extend this
198 application?
199 The hard way
201 Maypole was written because I don't like writing more Perl code than is
202 necessary. I also don't like writing HTML. In fact, I don't really get
203 on this whole computer thing, to be honest. But we'll start by ways
204 that we can customize the beer application simply by adding methods or
205 changing properties of the Perl driver code.
206 The first thing we ought to look into is the names of the columns; most
208 of them are fine, but that "Abv" column stands out. I'd rather that was
209 "A.B.V.". Maypole uses the "column_names" method to map between the
210 names of the columns in the database to the names it displays in the
211 default templates. This is provided by Maypole::Model::Base, and
212 normally, it does a pretty good job; it turns "model_number" into
213 "Model Number", for instance, but there was no way it could guess that
214 "abv" was an abbreviation. Since it returns a hash, the easiest way to
215 correct it is to construct a hash consisting of the bits it got right,
216 and then override the bits it got wrong:
217 package BeerDB::Beer;
219 sub column_names {
220 (shift->SUPER::column_names(), abv => "A.B.V.")
221 }
222 There's something to be aware of here: where are you going to type that
224 code? You can just put it in BeerDB.pm. Perl will be happy with that,
225 though you might want to put an extra pair of braces around it to limit
226 the scope of that package declaration. Alternatively, you might think
227 it's neater to put it in a file called BeerDB/Beer.pm, which is the
228 natural home for the package. This would certainly be a good idea if
229 you have a lot of other code to add to the "BeerDB::Beer" package. But
230 if you do that, you will have to tell Perl to load the BeerDB/Beer.pm
231 file by adding a line to BeerDB.pm:
232 BeerDB::Beer->require;
234 For another example of customization, the order of columns is a bit
236 wonky. We can fix this by overriding the "display_columns" method; this
237 is also a good way to hide away any columns we don't want to have
238 displayed, in the same way as declaring the "display_tables"
239 configuration parameter let us hide away tables we weren't using:
240 sub display_columns {
242 ("name", "brewery", "style", "price", "score", "abv", "notes")
243 }
244 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 "BeerDB::Beer"
248 object can call "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 "BeerDB::Beer" class like so:
251 sub related { "pubs" }
253 Now when we view a beer, we'll have a list of the pubs that it's on at.
255 Links
257 Contents, Next The Request Cookbook, Previous Maypole's Request
258 Workflow
259perl v5.32.0 2020-07-28 Maypole::Manual::Beer(3)