1Maypole::Manual::Model(U3s)er Contributed Perl DocumentatMiaoynpole::Manual::Model(3)
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6 Maypole::Manual::Model - Maypole Model Classes
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9 Maypole's model classes provide an interface to your data store. In
10 principle Maypole can connect to pretty much any data source, but the
11 default model is based on the popular Class::DBI object interface that
12 uses the near-universal DBI Perl interface to databases.
13 Maypole::Model::CDBI
15 Maypole model classes are pretty evil. The very first thing a Maypole
17 model class will do in a Maypole application is to cause a load of ta‐
18 ble-based classes to come into being, and then assimilate them.
19 What I mean by this is that when you set up a Maypole application, in
21 your driver class, you'll say something like this:
22 BeerDB->setup("dbi:mysql:beerdb");
24 "setup" is a Maypole method, and it hands its parameter to the model
26 class. In our case, the argument is a DBI connect string, because
27 that's what "Maypole::Model::CDBI", the "Class::DBI"-based model
28 expects. "Maypole::Model::CDBI" has a method called "setup_database"
29 that creates all the "Class::DBI" table classes after connecting to the
30 database with that connect string. It does this by using
31 "Class::DBI::Loader", a utility module which asks a database about its
32 schema and sets up classes such as "BeerDB::Beer" to inherit from
33 "Class::DBI".
34 Now it gets interesting. The names of these classes are stashed away in
36 the application's configuration, and then Maypole forcibly has these
37 classes inherit from the model class. Our "BeerDB::Beer" now inherits
38 both from "Class::DBI" and "Maypole::Model::CDBI".
39 This is useful for two reasons. The first reason is that "May‐
41 pole::Model::CDBI" is stuffed full of "Class::DBI" goodies that make
42 writing Maypole applications a lot easier:
43 package Maypole::Model::CDBI;
45 use base qw(Maypole::Model::Base Class::DBI);
46 use Maypole::Model::CDBI::AsForm;
47 use Class::DBI::FromCGI; # probabyly broken .
48 use Class::DBI::Loader;
49 use Class::DBI::AbstractSearch;
50 use Class::DBI::Plugin::RetrieveAll;
51 use Class::DBI::Pager;
52 We'll meet most of these goodies in the Standard Templates and Actions
54 chapter, where we explain how "Maypole::Model::CDBI" works.
55 The second reason why we want our table classes to inherit from "May‐
57 pole::Model::CDBI" is because it provides a useful set of default
58 actions. So what's an action, and why are they useful?
59 Extending a model class with actions
61 Maypole operates primarily by turning URLs into method calls on a model
63 class. All that the model stage of Maypole's operation does, when it
64 comes down to it, is maintain a mapping of tables to classes, and
65 despatch a HTTP request to a method call on the relevant table class.
66 This means that if you request a URL such as
67 http://localhost/beerdb/brewery/delete/20
69 Maypole's first stage of operation is to turn that into "BeerDB::Brew‐
71 ery->delete(20)". Now, it's slightly more complex than that. Firstly
72 because it doesn't actually pass the parameter 20, but it passes an
73 object representing row 20 in the database, but we can gloss over that
74 for the second. No, the real issue is that Maypole does not allow you
75 to call just any method in the table class; that would be somewhat
76 insecure.
77 Instead, Maypole makes a distinction between the kind of methods that
79 only the class itself and other Perl code can call, and the kind of
80 methods that anyone can call from a URL. This latter set of methods are
81 called exported methods, and exporting is done by means of Perl
82 attributes. You define a method to be exported like so:
83 sub drink :Exported {
85 This will allow the user to access "/beerdb/beer/drink" over the web.
87 An exported method accompanied with a template to render its output is
88 sometimes called an action.
89 Maypole model classes like "Maypole::Model::CDBI" come with a rela‐
91 tively handy set of actions which are all you need to set up a CRUD
92 (Create, Read, Update, Delete) database front-end: viewing a row in a
93 database, editing it, adding a new one, deleting, and so on. The most
94 important thing about Maypole, though, is that it doesn't stop there.
95 You can add your own.
96 For instance, in our beer database application, we could create a
98 "BeerDB::Beer" package, and put some additional actions in there.
99 package BeerDB::Beer;
101 sub top_five :Exported {
102 my ($class, $r) = @_;
103 $r->objects([ ($r->retrieve_all_sorted_by("score"))[-5..-1] ]);
104 }
105 Our action is called as a class method with the Maypole request object.
107 It uses the "Class::DBI::Plugin::RetrieveAll" module's
108 "retrieve_all_sorted_by" mixin to get the top five scoring beers, and
109 puts these in the "objects" slot of the request of object. Next, the
110 view class will come along and process the "top_five" template with
111 these five beers.
112 We'll look more at how to put together actions in the Standard Tem‐
114 plates and Actions chapter and our case studies.
115 Links
117 Contents, Next Maypole View Classes, Previous Introduction to Maypole
119perl v5.8.8 2005-11-23 Maypole::Manual::Model(3)