NAME

Catalyst::Manual::Tutorial::MoreCatalystBasics - Catalyst Tutorial - Part 3: More Catalyst Application Development Basics

OVERVIEW

This is Part 3 of 10 for the Catalyst tutorial.

Tutorial Overview

DESCRIPTION

This part of the tutorial builds on the work done in Part 2 to explore some features that are more typical of "real world" web applications. From this part of the tutorial onward, we will be building a simple book database application. Although the application will be too limited to be of use to anyone, it should provide a basic environment where we can explore a variety of features used in virtually all web applications.

You can checkout the source code for this example from the catalyst subversion repository as per the instructions in Catalyst::Manual::Tutorial::Intro

CREATE A NEW APPLICATION

The remainder of the tutorial will build an application call MyApp. Use the Catalyst catalyst.pl script to initialize the framework for an application called MyApp (make sure you aren't still inside the directory of the Hello application from the previous part of the tutorial):

    $ catalyst.pl MyApp
    created "MyApp"
    created "MyApp/script"
    created "MyApp/lib"
    created "MyApp/root"
    ...
    created "MyApp/script/myapp_create.pl"
    $ cd MyApp

This creates a similar skeletal structure to what we saw in Part 2 of the tutorial, except with MyApp or myapp substituted for Hello and hello.

EDIT THE LIST OF CATALYST PLUGINS

One of the greatest benefits of Catalyst is that it has such a large library of plugins available. Plugins are used to seamlessly integrate existing Perl modules into the overall Catalyst framework. In general, they do this by adding additional methods to the context object (generally written as $c) that Catalyst passes to every component throughout the framework.

By default, Catalyst enables three plugins/flags:

-Debug Flag

Enables the Catalyst debug output you saw when we started the script/myapp_server.pl development server earlier. You can remove this plugin when you place your application into production.

As you may have noticed, -Debug is not a plugin, but a flag. Although most of the items specified on the use Catalyst line of your application class will be plugins, Catalyst supports a limited number of flag options (of these, -Debug is the most common). See the documentation for Catalyst.pm to get details on other flags (currently -Engine, -Home, and -Log).

If you prefer, you can use the $c->debug method to enable debug messages.

TIP: Depending on your needs, it can be helpful to permanently remove -Debug from lib/MyApp.pm and then use the -d option to script/myapp_server.pl to re-enable it just for the development server. We will not be using that approach in the tutorial, but feel free to make use of it in your own projects.
Catalyst::Plugin::ConfigLoader

ConfigLoader provides an automatic way to load configurable parameters for your application from a central Config::General file (versus having the values hard-coded inside your Perl modules). Config::General uses syntax very similar to Apache configuration files. We will see how to use this feature of Catalyst during the authentication and authorization sections (Part 5 and Part 6).

IMPORTANT NOTE: If you are following along in Ubuntu 8.04 or otherwise using a version of Catalyst::Devel prior to version 1.06, you need to be aware that Catalyst changed from a default format of YAML to the more straightforward Config::General format. Because Catalyst has long supported both formats, this tutorial will simply use a configuration file called myapp.conf instead of myapp.yml and Catatlyst will automcatically use the new format. Just be aware that earlier versions of Catalyst will still create the myapp.yml file and that you will need to remove myapp.yml and create a new myapp.conf file by hand, but otherwise this transition is very painless. The default contents of myapp.conf should only consist of one line: name MyApp. Also be aware that you can continue to use any format supported by Catalyst::Plugin::ConfigLoader and Config::Any, including YAML -- Catalyst will automatically look for any of the supported configuration file formats.

TIP: This script can be useful for converting between configuration formats:
```
    perl -Ilib -e 'use MyApp; use Config::General; 
        Config::General->new->save_file("myapp.conf", MyApp->config);'
```
NOTE: The default myapp.conf should look like:
```
    name   MyApp
```
Catalyst::Plugin::Static::Simple

Static::Simple provides an easy method of serving static content such as images and CSS files under the development server.

To modify the list of plugins, edit lib/MyApp.pm (this file is generally referred to as your application class) and delete the line with:

    use Catalyst qw/-Debug ConfigLoader Static::Simple/;

Replace it with:

    use Catalyst qw/
            -Debug
            ConfigLoader
            Static::Simple
            
            StackTrace
            /;

This tells Catalyst to start using one new plugin:

Catalyst::Plugin::StackTrace

Adds a stack trace to the standard Catalyst "debug screen" (this is the screen Catalyst sends to your browser when an error occurs).

Note: StackTrace output appears in your browser, not in the console window from which you're running your application, which is where logging output usually goes.

Note: You will want to disable StackTrace before you put your application into production, but it can be helpful during development.

Note that when specifying plugins on the use Catalyst line, you can omit Catalyst::Plugin:: from the name. Additionally, you can spread the plugin names across multiple lines as shown here, or place them all on one (or more) lines as with the default configuration.

CREATE A CATALYST CONTROLLER

As discussed earlier, controllers are where you write methods that interact with user input. Typically, controller methods respond to GET and POST messages from the user's web browser.

Use the Catalyst create script to add a controller for book-related actions:

    $ script/myapp_create.pl controller Books
     exists "/home/me/MyApp/script/../lib/MyApp/Controller"
     exists "/home/me/MyApp/script/../t"
    created "/home/me/MyApp/script/../lib/MyApp/Controller/Books.pm"
    created "/home/me/MyApp/script/../t/controller_Books.t"

Then edit lib/MyApp/Controller/Books.pm and add the following method to the controller:

    =head2 list
    
    Fetch all book objects and pass to books/list.tt2 in stash to be displayed
    
    =cut
     
    sub list : Local {
        # Retrieve the usual Perl OO '$self' for this object. $c is the Catalyst
        # 'Context' that's used to 'glue together' the various components
        # that make up the application
        my ($self, $c) = @_;
    
        # Retrieve all of the book records as book model objects and store in the
        # stash where they can be accessed by the TT template
        $c->stash->{books} = [$c->model('DB::Books')->all];
        
        # Set the TT template to use.  You will almost always want to do this
        # in your action methods (action methods respond to user input in
        # your controllers).
        $c->stash->{template} = 'books/list.tt2';
    }

Note: This won't actually work yet since you haven't set up your model yet. We will be covering the model soon.

Note: Programmers experienced with object-oriented Perl should recognize $self as a reference to the object where this method was called. On the other hand, $c will be new to many Perl programmers who have not used Catalyst before (it's sometimes written as $context). The Context object is automatically passed to all Catalyst components. It is used to pass information between components and provide access to Catalyst and plugin functionality.

TIP: You may see the $c->model('DB::Book') used above written as $c->model('DB')->resultset('Book). The two are equivalent.

Note: Catalyst actions are regular Perl methods, but they make use of Nicholas Clark's attributes module (that's the : Local next to the sub list in the code above) to provide additional information to the Catalyst dispatcher logic. Many newer Catalyst applications are switching to the use of "Literal" :Path actions and Args attribute in lieu of : Local and : Private. For example, sub any_method :Path :Args(0) can be used instead of sub index :Private (because no path was supplied to Path it matches the "empty" URL in the namespace of that module... the same thing sub index would do) or sub list :Path('list') :Args(0) could be used instead of the sub list : Local above (the list argument to Path would make it match on the URL list under books, the namespace of the current module). See "Action Types" in Catalyst::Manual::Intro as well as Part 5 of this tutorial (Authentication) for additional information. Another popular but more advanced feature is Chained actions that allow a single URL to "chain together" multiple action method calls, each with an appropriate number of arguments (see Catalyst::DispatchType::Chained for details).

CATALYST VIEWS

As mentioned in Part 2 of the tutorial, views are where you render output, typically for display in the user's web browser, but also possibly using other display output- generation systems. As with virtually every aspect of Catalyst, options abound when it comes to the specific view technology you adopt inside your application. However, most Catalyst applications use the Template Toolkit, known as TT (for more information on TT, see http://www.template-toolkit.org). Other popular view technologies include Mason (http://www.masonhq.com and http://www.masonbook.com) and HTML::Template ("/html- template.sourceforge.net" in http:).

Create a Catalyst View Using `TTSite`

When using TT for the Catalyst view, there are two main helper scripts:

Both are similar, but TT merely creates the lib/MyApp/View/TT.pm file and leaves the creation of any hierarchical template organization entirely up to you. (It also creates a t/view_TT.t file for testing; test cases will be discussed in Part 8). The TTSite helper creates a modular and hierarchical view layout with separate Template Toolkit (TT) files for common header and footer information, configuration values, a CSS stylesheet, and more.

While TTSite is useful to bootstrap a project, we recommend that unless you know what you're doing or want to pretty much use the supplied templates as is, that you use the plain Template Toolkit view when starting a project from scratch. This is because TTSite can be tricky to customize. Additionally TT contains constructs that you need to learn yourself if you're going to be a serious user of TT. Our experience suggests that you're better off learning these from scratch. We use TTSite here precisely because it is useful for bootstrap/prototype purposes.

Enter the following command to enable the TTSite style of view rendering for this tutorial:

    $ script/myapp_create.pl view TT TTSite
     exists "/home/me/MyApp/script/../lib/MyApp/View"
     exists "/home/me/MyApp/script/../t"
    created "/home/me/MyApp/script/../lib/MyApp/View/TT.pm"
    created "/home/me/MyApp/script/../root/lib"
    ...
    created "/home/me/MyApp/script/../root/src/ttsite.css"

This puts a number of files in the root/lib and root/src directories that can be used to customize the look and feel of your application. Also take a look at lib/MyApp/View/TT.pm for config values set by the TTSite helper.

TIP: Note that TTSite does one thing that could confuse people who are used to the normal TT Catalyst view: it redefines the Catalyst context object in templates from its usual c to Catalyst. When looking at other Catalyst examples, remember that they almost always use c. Note that Catalyst and TT do not complain when you use the wrong name to access the context object...TT simply outputs blanks for that bogus logic (see next tip to change this behavior with TT DEBUG options). Finally, be aware that this change in name only applies to how the context object is accessed inside your TT templates; your controllers will continue to use $c (or whatever name you use when fetching the reference from @_ inside your methods). (You can change back to the "default" behavior be removing the CATALYST_VAR line from lib/MyApp/View/TT.pm, but you will also have to edit root/lib/config/main and root/lib/config/url. If you do this, be careful not to have a collision between your own c variable and the Catalyst c variable.)

TIP: When troubleshooting TT it can be helpful to enable variable DEBUG options. You can do this in a Catalyst environment by adding a DEBUG line to the __PACKAGE__-config> declaration in lib/MyApp/View/TT.pm:

    __PACKAGE__->config({
        CATALYST_VAR => 'Catalyst',
        ...
        DEBUG        => 'undef',
        ...
    });

Note: __PACKAGE__ is just a shorthand way of referencing the name of the package where it is used. Therefore, in TT.pm, __PACKAGE__ is equivalent to TT.

There are a variety of options you can use, such as 'undef', 'all', 'service', 'context', 'parser', 'provider', and 'service'. See Template::Constants for more information (remove the DEBUG_ portion of the name shown in the TT docs and convert to lower case for use inside Catalyst).

NOTE: Please be sure to disable TT debug options before continuing the tutorial (especially the 'undef' option -- leaving this enabled will conflict with several of the conventions used by this tutorial and TTSite to leave some variables undefined on purpose).

Globally Customize Every View

When using TTSite, files in the subdirectories of root/lib can be used to make changes that will appear in every view. For example, to display optional status and error messages in every view, edit root/lib/site/layout, updating it to match the following (the two HTML span elements are new):

    <div id="header">[% PROCESS site/header %]</div>
    
    <div id="content">
    <span class="message">[% status_msg %]</span>
    <span class="error">[% error_msg %]</span>
    [% content %]
    </div>
    
    <div id="footer">[% PROCESS site/footer %]</div>

If we set either message in the Catalyst stash (e.g., $c->stash->{status_msg} = 'Request was successful!') it will be displayed whenever any view used by that request is rendered. The message and error CSS styles are automatically defined in root/src/ttsite.css and can be customized to suit your needs.

Note: The Catalyst stash only lasts for a single HTTP request. If you need to retain information across requests you can use Catalyst::Plugin::Session (we will use Catalyst sessions in the Authentication part of the tutorial).

Create a TT Template Page

To add a new page of content to the TTSite view hierarchy, just create a new .tt2 file in root/src. Only include HTML markup that goes inside the HTML <body> and </body> tags, TTSite will use the contents of root/lib/site to add the top and bottom.

First create a directory for book-related TT templates:

    $ mkdir root/src/books

Then create root/src/books/list.tt2 in your editor and enter:

    [% # This is a TT comment.  The '-' at the end "chomps" the newline.  You won't -%]
    [% # see this "chomping" in your browser because HTML ignores blank lines, but  -%]
    [% # it WILL eliminate a blank line if you view the HTML source.  It's purely   -%]
    [%- # optional, but both the beginning and the ending TT tags support chomping. -%]
    
    [% # Provide a title to root/lib/site/header -%]
    [% META title = 'Book List' -%]
    
    <table>
    <tr><th>Title</th><th>Rating</th><th>Author(s)</th></tr>
    [% # Display each book in a table row %]
    [% FOREACH book IN books -%]
      <tr>
        <td>[% book.title %]</td>
        <td>[% book.rating %]</td>
      </tr>
    [% END -%]
    </table>

As indicated by the inline comments above, the META title line uses TT's META feature to provide a title to root/lib/site/header. Meanwhile, the outer FOREACH loop iterates through each book model object and prints the title and rating fields. An inner FOREACH loop prints the last name of each author in a comma-separated list within a single table cell.

If you are new to TT, the [% and %] tags are used to delimit TT code. TT supports a wide variety of directives for "calling" other files, looping, conditional logic, etc. In general, TT simplifies the usual range of Perl operators down to the single dot (.) operator. This applies to operations as diverse as method calls, hash lookups, and list index values (see http://www.template-toolkit.org/docs/default/Manual/Variables.html for details and examples). In addition to the usual Template module Pod documentation, you can access the TT manual at http://www.template-toolkit.org/docs/default/.

NOTE: The TTSite helper creates several TT files using an extension of .tt2. Most other Catalyst and TT examples use an extension of .tt. You can use either extension (or no extension at all) with TTSite and TT, just be sure to use the appropriate extension for both the file itself and the $c->stash->{template} = ... line in your controller. This document will use .tt2 for consistency with the files already created by the TTSite helper.

CREATE A SQLITE DATABASE

In this step, we make a text file with the required SQL commands to create a database table and load some sample data. Open myapp01.sql in your editor and enter:

    --
    -- Create a very simple database to hold book and author information
    --
    CREATE TABLE books (
            id          INTEGER PRIMARY KEY,
            title       TEXT ,
            rating      INTEGER
    );
    -- 'book_authors' is a many-to-many join table between books & authors
    CREATE TABLE book_authors (
            book_id     INTEGER,
            author_id   INTEGER,
            PRIMARY KEY (book_id, author_id)
    );
    CREATE TABLE authors (
            id          INTEGER PRIMARY KEY,
            first_name  TEXT,
            last_name   TEXT
    );
    ---
    --- Load some sample data
    ---
    INSERT INTO books VALUES (1, 'CCSP SNRS Exam Certification Guide', 5);
    INSERT INTO books VALUES (2, 'TCP/IP Illustrated, Volume 1', 5);
    INSERT INTO books VALUES (3, 'Internetworking with TCP/IP Vol.1', 4);
    INSERT INTO books VALUES (4, 'Perl Cookbook', 5);
    INSERT INTO books VALUES (5, 'Designing with Web Standards', 5);
    INSERT INTO authors VALUES (1, 'Greg', 'Bastien');
    INSERT INTO authors VALUES (2, 'Sara', 'Nasseh');
    INSERT INTO authors VALUES (3, 'Christian', 'Degu');
    INSERT INTO authors VALUES (4, 'Richard', 'Stevens');
    INSERT INTO authors VALUES (5, 'Douglas', 'Comer');
    INSERT INTO authors VALUES (6, 'Tom', 'Christiansen');
    INSERT INTO authors VALUES (7, 'Nathan', 'Torkington');
    INSERT INTO authors VALUES (8, 'Jeffrey', 'Zeldman');
    INSERT INTO book_authors VALUES (1, 1);
    INSERT INTO book_authors VALUES (1, 2);
    INSERT INTO book_authors VALUES (1, 3);
    INSERT INTO book_authors VALUES (2, 4);
    INSERT INTO book_authors VALUES (3, 5);
    INSERT INTO book_authors VALUES (4, 6);
    INSERT INTO book_authors VALUES (4, 7);
    INSERT INTO book_authors VALUES (5, 8);

TIP: See Appendix 1 for tips on removing the leading spaces when cutting and pasting example code from POD-based documents.

Then use the following command to build a myapp.db SQLite database:

    $ sqlite3 myapp.db < myapp01.sql

If you need to create the database more than once, you probably want to issue the rm myapp.db command to delete the database before you use the sqlite3 myapp.db < myapp01.sql command.

Once the myapp.db database file has been created and initialized, you can use the SQLite command line environment to do a quick dump of the database contents:

    $ sqlite3 myapp.db
    SQLite version 3.4.2
    Enter ".help" for instructions
    sqlite> select * from books;
    1|CCSP SNRS Exam Certification Guide|5
    2|TCP/IP Illustrated, Volume 1|5
    3|Internetworking with TCP/IP Vol.1|4
    4|Perl Cookbook|5
    5|Designing with Web Standards|5
    sqlite> .q
    $

Or:

    $ sqlite3 myapp.db "select * from books"
    1|CCSP SNRS Exam Certification Guide|5
    2|TCP/IP Illustrated, Volume 1|5
    3|Internetworking with TCP/IP Vol.1|4
    4|Perl Cookbook|5
    5|Designing with Web Standards|5

As with most other SQL tools, if you are using the full "interactive" environment you need to terminate your SQL commands with a ";" (it's not required if you do a single SQL statement on the command line). Use ".q" to exit from SQLite from the SQLite interactive mode and return to your OS command prompt.

DATABASE ACCESS WITH `DBIx::Class`

Catalyst can be used with virtually any form of persistent datastore available via Perl. For example, Catalyst::Model::DBI can be used to easily access databases through the traditional Perl DBI interface. However, most Catalyst applications use some form of ORM technology to automatically create and save model objects as they are used. Although Tony Bowden's Class::DBI has been a popular choice in the past, Matt Trout's DBIx::Class (abbreviated as "DBIC") has rapidly emerged as the Perl-based ORM technology of choice. Most new Catalyst applications rely on DBIC, as will this tutorial.

Create a dynamic DBIC Model

Use the create=dynamic model helper option to build a model that dynamically reads your database structure every time the application starts:

    $ script/myapp_create.pl model DB DBIC::Schema MyApp::Schema create=dynamic dbi:SQLite:myapp.db
     exists "/home/kclark/dev/MyApp/script/../lib/MyApp/Model"
     exists "/home/kclark/dev/MyApp/script/../t"
     exists "/home/kclark/dev/MyApp/script/../lib/MyApp"
    created "/home/kclark/dev/MyApp/script/../lib/MyApp/Schema.pm"
    created "/home/kclark/dev/MyApp/script/../lib/MyApp/Model/DB.pm"
    created "/home/kclark/dev/MyApp/script/../t/model_DB.t"

DB is the name of the model class to be created by the helper in lib/MyApp/Model (Catalyst has a separate directory under lib/MyApp for each of the three parts of MVC: Model, View, and Controller). DBIC::Schema is the type of the model to create. MyApp::Schema is the name of the DBIC schema file written to lib/MyApp/Schema.pm. Because we specified create=dynamic to the helper, it use DBIx::Class::Schema::Loader to dynamically load the schema information from the database every time the application starts. And finally, dbi:SQLite:myapp.db is the standard DBI connect string for use with SQLite.

NOTE: Although the create=dynamic option to the DBIC helper makes for a nifty demonstration, is only really suitable for very small applications. After this demonstration, you should almost always use the create=static option that we switch to below.

RUN THE APPLICATION

First, let's enable an environment variable option that causes DBIx::Class to dump the SQL statements it's using to access the database (this option can provide extremely helpful troubleshooting information):

    $ export DBIC_TRACE=1

This assumes you are using BASH as your shell -- adjust accordingly if you are using a different shell (for example, under tcsh, use setenv DBIC_TRACE 1).

NOTE: You can also set this in your code using $class->storage->debug(1);. See DBIx::Class::Manual::Troubleshooting for details (including options to log to file instead of displaying to the Catalyst development server log).

Then run the Catalyst "demo server" script:

    $ script/myapp_server.pl

Your development server log output should display something like:

    $script/myapp_server.pl
    [debug] Debug messages enabled
    [debug] Loaded plugins:
    .----------------------------------------------------------------------------.
    | Catalyst::Plugin::ConfigLoader  0.17                                       |
    | Catalyst::Plugin::StackTrace  0.06                                         |
    | Catalyst::Plugin::Static::Simple  0.20                                     |
    '----------------------------------------------------------------------------'
    
    [debug] Loaded dispatcher "Catalyst::Dispatcher"
    [debug] Loaded engine "Catalyst::Engine::HTTP"
    [debug] Found home "/home/me/MyApp"
    [debug] Loaded Config "/home/me/MyApp/myapp.conf"
    [debug] Loaded components:
    .-----------------------------------------------------------------+----------.
    | Class                                                           | Type     |
    +-----------------------------------------------------------------+----------+
    | MyApp::Controller::Books                                        | instance |
    | MyApp::Controller::Root                                         | instance |
    | MyApp::Model::DB                                                | instance |
    | MyApp::Model::DB::Authors                                       | class    |
    | MyApp::Model::DB::BookAuthors                                   | class    |
    | MyApp::Model::DB::Books                                         | class    |
    | MyApp::View::TT                                                 | instance |
    '-----------------------------------------------------------------+----------'
    
    [debug] Loaded Private actions:
    .----------------------+--------------------------------------+--------------.
    | Private              | Class                                | Method       |
    +----------------------+--------------------------------------+--------------+
    | /default             | MyApp::Controller::Root              | default      |
    | /end                 | MyApp::Controller::Root              | end          |
    | /books/index         | MyApp::Controller::Books             | index        |
    | /books/list          | MyApp::Controller::Books             | list         |
    '----------------------+--------------------------------------+--------------'
    
    [debug] Loaded Path actions:
    .-------------------------------------+--------------------------------------.
    | Path                                | Private                              |
    +-------------------------------------+--------------------------------------+
    | /books/list                         | /books/list                          |
    '-------------------------------------+--------------------------------------'
    
    [info] MyApp powered by Catalyst 5.7011
    You can connect to your server at http://localhost:3000

NOTE: Be sure you run the script/myapp_server.pl command from the 'base' directory of your application, not inside the script directory itself or it will not be able to locate the myapp.db database file. You can use a fully qualified or a relative path to locate the database file, but we did not specify that when we ran the model helper earlier.

Some things you should note in the output above:

Catalyst::Model::DBIC::Schema dynamically created three model classes, one to represent each of the three tables in our database (MyApp::Model::DB::Authors, MyApp::Model::DB::BookAuthors, and MyApp::Model::DB::Books).
The "list" action in our Books controller showed up with a path of /books/list.

Point your browser to http://localhost:3000 and you should still get the Catalyst welcome page.

Next, to view the book list, change the URL in your browser to http://localhost:3000/books/list. You should get a list of the five books loaded by the myapp01.sql script above, with TTSite providing the formatting for the very simple output we generated in our template. The rating for each book should appear on each row.

Also notice in the output of the script/myapp_server.pl that DBIC used the following SQL to retrieve the data:

    SELECT me.id, me.title, me.rating FROM books me

because we enabled DBIC_TRACE.

You now the beginnings of a simple but workable web application. Continue on to future sections and we will develop the application more fully.

A STATIC DATABASE MODEL WITH `DBIx::Class`

Create Static DBIC Schema Files

Unlike the previous section where we had DBIC automatically discover the structure of the database every time the application started, here we will use static schema files for more control. This is typical of most "real world" applications.

One option would be to create a separate schema file for each table in the database, however, lets use the same DBIx::Class::Schema::Loader used earlier with create=dynamic to build the static files for us. First, lets remove the schema file created earlier:

    $ rm lib/MyApp/Schema.pm

Now regenerate the schema using the create=static option:

    $ script/myapp_create.pl model DB DBIC::Schema MyApp::Schema create=static dbi:SQLite:myapp.db
     exists "/home/kclark/dev/MyApp/script/../lib/MyApp/Model"
     exists "/home/kclark/dev/MyApp/script/../t"
    Dumping manual schema for MyApp::Schema to directory /home/kclark/dev/MyApp/script/../lib ...
    Schema dump completed.
     exists "/home/kclark/dev/MyApp/script/../lib/MyApp/Model/DB.pm"

We could have also deleted lib/MyApp/Model/DB.pm, but it would have regenerated the same file (note the exists in the output above). If you take a look at lib/MyApp/Model/DB.pm, it simply contains a reference to the actual schema file in lib/MyApp/Schema.pm along with the database connect string.

If you look in the lib/MyApp/Schema directory, you will find that DB.pm is no longer using DBIx::Class::Schema::Loader as its base class (DBIx::Class::Schema::Loader is only being used by the helper to load the schema once and then create the static files for us) and that it only contains a call to the load_classes method. You will also find that lib/MyApp/Schema contains a Schema subdirectory, with one file inside this directory for each of the tables in our simple database (Authors.pm, BookAuthors.pm, and Books.pm). These three files were created based on the information found by DBIx::Class::Schema::Loader as the helper ran.

The idea with all of the files created under lib/MyApp/Schema by the create=static option is to only edit the files below the # DO NOT MODIFY THIS OR ANYTHING ABOVE! warning. If you place all of your changes below that point in the file, you can regenerate the auto-generated information at the top of each file should your database structure get updated.

Also note the "flow" of the model information across the various files and directories. Catalyst will initially load the model from lib/MyApp/Model/DB.pm. This file contains a reference to lib/MyApp/Schema.pm, so that file is loaded next. Finally, the call to load_classes in that file will load each of the table-specific "results source" files from the lib/MyApp/Schema subdirectory. These three table-specific DBIC schema files will then be used to create three table-specific Catalyst models every time the application starts (you can see these three model files listed in the debug output generated when you launch the application).

Updating the Generated DBIC Schema Files

Let's manually add some relationship information to the auto-generated schema files. First edit lib/MyApp/Schema/Books.pm and add the following text below the # You can replace this text... comment:

    #
    # Set relationships:
    #   
    
    # has_many():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of the model class referenced by this relationship
    #     3) Column name in *foreign* table
    __PACKAGE__->has_many(book_authors => 'MyApp::Schema::BookAuthors', 'book_id');
    
    # many_to_many():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of has_many() relationship this many_to_many() is shortcut for 
    #     3) Name of belongs_to() relationship in model class of has_many() above 
    #   You must already have the has_many() defined to use a many_to_many().
    __PACKAGE__->many_to_many(authors => 'book_authors', 'author');

Note: Be careful to put this code above the 1; at the end of the file. As with any Perl package, we need to end the last line with a statement that evaluates to true. This is customarily done with 1; on a line by itself.

This code defines both a has_many and a many_to_many relationship. The many_to_many relationship is optional, but it makes it easier to map a book to its collection of authors. Without it, we would have to "walk" though the book_authors table as in $book->book_authors- >first->author->last_name (we will see examples on how to use DBIC objects in your code soon, but note that because $book- >book_authors can return multiple authors, we have to use first to display a single author). many_to_many allows us to use the shorter $book->authors->first->last_name. Note that you cannot define a many_to_many relationship without also having the has_many relationship in place.

Then edit lib/MyApp/Schema/Authors.pm and add relationship information as follows (again, be careful to put in above the 1; but below the # DO NOT MODIFY THIS OR ANYTHING ABOVE! comment):

    #
    # Set relationships:
    #
    
    # has_many():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of the model class referenced by this relationship
    #     3) Column name in *foreign* table
    __PACKAGE__->has_many(book_author => 'MyApp::Schema::BookAuthors', 'author_id');
    
    # many_to_many():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of has_many() relationship this many_to_many() is shortcut for
    #     3) Name of belongs_to() relationship in model class of has_many() above 
    #   You must already have the has_many() defined to use a many_to_many().
    __PACKAGE__->many_to_many(books => 'book_author', 'book');

Finally, do the same for the "join table," lib/MyApp/Schema/BookAuthors.pm:

    #
    # Set relationships:
    #
    
    # belongs_to():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of the model class referenced by this relationship
    #     3) Column name in *this* table
    __PACKAGE__->belongs_to(book => 'MyApp::Schema::Books', 'book_id');
    
    # belongs_to():
    #   args:
    #     1) Name of relationship, DBIC will create accessor with this name
    #     2) Name of the model class referenced by this relationship
    #     3) Column name in *this* table
    __PACKAGE__->belongs_to(author => 'MyApp::Schema::Authors', 'author_id');

RUN THE APPLICATION

Run the Catalyst "demo server" script with the DBIC_TRACE option (it might still be enabled from earlier in the tutorial, but here is an alternate way to specify the option just in case):

    $ DBIC_TRACE=1 script/myapp_server.pl

Make sure that the application loads correctly and that you see the three dynamically created model class (one for each of the table-specific schema classes we created).

Then hit the URL http://localhost:3000/books/list and be sure that the book list is displayed.

RUNNING THE APPLICATION FROM THE COMMAND LINE

In some situations, it can be useful to run your application and display a page without using a browser. Catalyst lets you do this using the scripts/myapp_test.pl script. Just supply the URL you wish to display and it will run that request through the normal controller dispatch logic and use the appropriate view to render the output (obviously, complex pages may dump a lot of text to your terminal window). For example, if you type:

    $ script/myapp_test.pl "/books/list"

You should get the same text as if you visited http://localhost:3000/books/list with the normal development server and asked your browser to view the page source.

UPDATING THE VIEW

Let's add a new column to our book list page that takes advantage of the relationship information we manually added to our schema files in the previous section. Edit root/src/books/list.tt2 add add the following code below the existing table cell that contains book.rating (IOW, add a new table cell below the existing two td cells):

    <td>
      [% # First initialize a TT variable to hold a list.  Then use a TT FOREACH -%]
      [% # loop in 'side effect notation' to load just the last names of the     -%]
      [% # authors into the list.  Note that the 'push' TT vmethod does not      -%]
      [% # a value, so nothing will be printed here.  But, if you have something -%]
      [% # in TT that does return a method and you don't want it printed, you    -%]
      [% # can: 1) assign it to a bogus value, or 2) use the CALL keyword to     -%]
      [% # call it and discard the return value.                                 -%]
      [% tt_authors = [ ];
         tt_authors.push(author.last_name) FOREACH author = book.authors %]
      [% # Now use a TT 'virtual method' to display the author count in parens   -%]
      [% # Note the use of the TT filter "| html" to escape dangerous characters -%]
      ([% tt_authors.size | html %])
      [% # Use another TT vmethod to join & print the names & comma separators   -%]
      [% tt_authors.join(', ') | html %]
    </td>

Then hit Ctrl+R in your browser (not that you don't need to reload the development server or use the -r option when updating TT templates) and you should now the the number of authors each book and a comma-separated list of the author's last names.

If you are still running the development server with DBIC_TRACE enabled, you should also now see five more SELECT statements in the debug output (one for each book as the authors are being retrieved by DBIC).

Also note that we are using "| html", a type of TT filter, to escape characters such as < and > to < and > and avoid various types of dangerous hacks against your application. In a real application, you would probably want to put "| html" at the end of every field where a user has control over the information that can appear in that field (and can therefore inject markup or code if you don't "neutralize" those fields). In addition to "| html", Template Toolkit has a variety of other useful filters that can found in the documentation for Template::Filters.

Using `RenderView` for the Default View

NOTE: The rest of this part of the tutorial is optional. You can skip to Part 4, Basic CRUD, if you wish.

Once your controller logic has processed the request from a user, it forwards processing to your view in order to generate the appropriate response output. Catalyst uses Catalyst::Action::RenderView by default to automatically performs this operation. If you look in lib/MyApp/Controller/Root.pm, you should see the empty definition for the sub end method:

    sub end : ActionClass('RenderView') {}

The following bullet points provide a quick overview of the RenderView process:

Root.pm is designed to hold application-wide logic.
At the end of a given user request, Catalyst will call the most specific end method that's appropriate. For example, if the controller for a request has an end method defined, it will be called. However, if the controller does not define a controller-specific end method, the "global" end method in Root.pm will be called.
Because the definition includes an ActionClass attribute, the Catalyst::Action::RenderView logic will be executed after any code inside the definition of sub end is run. See Catalyst::Manual::Actions for more information on ActionClass.
Because sub end is empty, this effectively just runs the default logic in RenderView. However, you can easily extend the RenderView logic by adding your own code inside the empty method body ({}) created by the Catalyst Helpers when we first ran the catalyst.pl to initialize our application. See Catalyst::Action::RenderView for more detailed information on how to extended RenderView in sub end.

Using The Default Template Name

By default, Catalyst::View::TT will look for a template that uses the same name as your controller action, allowing you to save the step of manually specifying the template name in each action. For example, this would allow us to remove the $c->stash->{template} = 'books/list.tt2'; line of our list action in the Books controller. Open lib/MyApp/Controller/Books.pm in your editor and comment out this line to match the following (only the $c->stash->{template} line has changed):

    =head2 list
    
    Fetch all book objects and pass to books/list.tt2 in stash to be displayed
    
    =cut
    
    sub list : Local {
        # Retrieve the usual Perl OO '$self' for this object. $c is the Catalyst
        # 'Context' that's used to 'glue together' the various components
        # that make up the application
        my ($self, $c) = @_;
    
        # Retrieve all of the book records as book model objects and store in the
        # stash where they can be accessed by the TT template
        $c->stash->{books} = [$c->model('DB::Books')->all];
    
        # Set the TT template to use.  You will almost always want to do this
        # in your action methods (actions methods respond to user input in
        # your controllers).
        #$c->stash->{template} = 'books/list.tt2';
    }

Catalyst::View::TT defaults to looking for a template with no extension. In our case, we need to override this to look for an extension of .tt2. Open lib/MyApp/View/TT.pm and add the TEMPLATE_EXTENSION definition as follows:

    __PACKAGE__->config({
        CATALYST_VAR => 'Catalyst',
        INCLUDE_PATH => [
            MyApp->path_to( 'root', 'src' ),
            MyApp->path_to( 'root', 'lib' )
        ],
        PRE_PROCESS  => 'config/main',
        WRAPPER      => 'site/wrapper',
        ERROR        => 'error.tt2',
        TIMER        => 0,
        TEMPLATE_EXTENSION => '.tt2',
    });

You should now be able to restart the development server as per the previous section and access the http://localhost:3000/books/list as before.

NOTE: Please note that if you use the default template technique, you will not be able to use either the $c->forward or the $c->detach mechanisms (these are discussed in Part 2 and Part 9 of the Tutorial).

Return To A Manually-Specified Template

In order to be able to use $c->forward and $c->detach later in the tutorial, you should remove the comment from the statement in sub list in lib/MyApp/Controller/Books.pm:

    $c->stash->{template} = 'books/list.tt2';

Then delete the TEMPLATE_EXTENSION line in lib/MyApp/View/TT.pm.

You should then be able to restart the development server and access http://localhost:3000/books/list in the same manner as with earlier sections.

AUTHOR

Kennedy Clark, hkclark@gmail.com

Please report any errors, issues or suggestions to the author. The most recent version of the Catalyst Tutorial can be found at http://dev.catalyst.perl.org/repos/Catalyst/trunk/Catalyst-Manual/lib/Catalyst/Manual/Tutorial/.

To install Catalyst::Manual, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Catalyst::Manual

CPAN shell

perl -MCPAN -e shell
install Catalyst::Manual

For more information on module installation, please visit the detailed CPAN module installation guide.

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NAME

OVERVIEW

DESCRIPTION

CREATE A NEW APPLICATION

EDIT THE LIST OF CATALYST PLUGINS

CREATE A CATALYST CONTROLLER

CATALYST VIEWS

Create a Catalyst View Using TTSite

Globally Customize Every View

Create a TT Template Page

CREATE A SQLITE DATABASE

DATABASE ACCESS WITH DBIx::Class

Create a dynamic DBIC Model

RUN THE APPLICATION

A STATIC DATABASE MODEL WITH DBIx::Class

Create Static DBIC Schema Files

Updating the Generated DBIC Schema Files

RUN THE APPLICATION

RUNNING THE APPLICATION FROM THE COMMAND LINE

UPDATING THE VIEW

Using RenderView for the Default View

Using The Default Template Name

Return To A Manually-Specified Template

AUTHOR

Module Install Instructions

Create a Catalyst View Using `TTSite`

DATABASE ACCESS WITH `DBIx::Class`

A STATIC DATABASE MODEL WITH `DBIx::Class`

Using `RenderView` for the Default View