Remove recurrent newbie mistake from tutorial

[catagits/Catalyst-Manual.git] / lib / Catalyst / Manual / Tutorial / 03_MoreCatalystBasics.pod

=head1 NAME

Catalyst::Manual::Tutorial::03_MoreCatalystBasics - Catalyst Tutorial - Chapter 3: More Catalyst Application Development Basics


=head1 OVERVIEW

This is B<Chapter 3 of 10> for the Catalyst tutorial.

L<Tutorial Overview|Catalyst::Manual::Tutorial>

=over 4

=item 1

L<Introduction|Catalyst::Manual::Tutorial::01_Intro>

=item 2

L<Catalyst Basics|Catalyst::Manual::Tutorial::02_CatalystBasics>

=item 3

B<03_More Catalyst Basics>

=item 4

L<Basic CRUD|Catalyst::Manual::Tutorial::04_BasicCRUD>

=item 5

L<Authentication|Catalyst::Manual::Tutorial::05_Authentication>

=item 6

L<Authorization|Catalyst::Manual::Tutorial::06_Authorization>

=item 7

L<Debugging|Catalyst::Manual::Tutorial::07_Debugging>

=item 8

L<Testing|Catalyst::Manual::Tutorial::08_Testing>

=item 9

L<Advanced CRUD|Catalyst::Manual::Tutorial::09_AdvancedCRUD>

=item 10

L<Appendices|Catalyst::Manual::Tutorial::10_Appendices>

=back


=head1 DESCRIPTION

This chapter of the tutorial builds on the work done in Chapter 2 to 
explore some features that are more typical of "real world" web 
applications. From this chapter 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 check out the source code for this example from the Catalyst
Subversion repository as per the instructions in
L<Catalyst::Manual::Tutorial::01_Intro|Catalyst::Manual::Tutorial::01_Intro>.

Please take a look at 
L<Catalyst::Manual::Tutorial::01_Intro/CATALYST INSTALLATION> before 
doing the rest of this tutorial.  Although the tutorial should work 
correctly under most any recent version of Perl running on any 
operating system, the tutorial has been written using Debian 5 and 
tested to be sure it runs correctly in this environment.  


=head1 CREATE A NEW APPLICATION

The remainder of the tutorial will build an application called C<MyApp>.
First use the Catalyst C<catalyst.pl> script to initialize the framework
for the C<MyApp> application (make sure you aren't still inside the
directory of the C<Hello> application from the previous chapter of the
tutorial or in a directory that already has a "MyApp" subdirectory):

    $ catalyst.pl MyApp
    created "MyApp"
    created "MyApp/script"
    created "MyApp/lib"
    created "MyApp/root"
    ...
    created "MyApp/script/myapp_create.pl"
    Change to application directory and Run "perl Makefile.PL" to make sure your install is complete
    $ cd MyApp

This creates a similar skeletal structure to what we saw in Chapter 2 of
the tutorial, except with C<MyApp> and C<myapp> substituted for
C<Hello> and C<hello>.  (As noted in Chapter 2, omit the ".pl" from 
the command if you are using Strawberry Perl.)


=head1 EDIT THE LIST OF CATALYST PLUGINS

One of the greatest benefits of Catalyst is that it has such a large 
library of bases classes and plugins available that you can use easily 
add functionality to your application. 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 C<context> 
object (generally written as C<$c>) that Catalyst passes to every 
component throughout the framework. 


By default, Catalyst enables three plugins/flags:

=over 4

=item *

C<-Debug> Flag

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

To be technically correct, it turns out that C<-Debug> is not a plugin, but a I<flag>. 
Although most of the items specified on the C<__PACKAGE__-E<gt>setup> 
line of your application class will be plugins, Catalyst supports a 
limited number of flag options (of these, C<-Debug> is the most 
common).  See the documentation for C<Catalyst.pm> to get details on 
other flags (currently C<-Engine>, C<-Home>, and C<-Log>).

If you prefer, there are several other ways to enable debug output:

=over 4

=item *

Use the C<$c-E<gt>debug> method

=item *

The C<-d> option to C<script/myapp_server.pl>

=item *

The C<CATALYST_DEBUG=1> environment variable (or set it to
zero to temporarily disable debug output).

=back

B<TIP>: Depending on your needs, it can be helpful to permanently
remove C<-Debug> from C<lib/MyApp.pm> and then use the C<-d> option
to C<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.

=item *

L<Catalyst::Plugin::ConfigLoader|Catalyst::Plugin::ConfigLoader>

C<ConfigLoader> provides an automatic way to load configurable
parameters for your application from a central
L<Config::General|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 (Chapter 5 and Chapter 6).

B<IMPORTANT NOTE:> If you are using a version of 
L<Catalyst::Devel|Catalyst::Devel> prior to version 1.06, be aware 
that Catalyst changed the default format from YAML to the more 
straightforward C<Config::General> style.  This tutorial uses the 
newer C<myapp.conf> file for C<Config::General>. However, Catalyst 
supports both formats and will automatically use either C<myapp.conf> 
or C<myapp.yml> (or any other format supported by 
L<Catalyst::Plugin::ConfigLoader|Catalyst::Plugin::ConfigLoader> and 
L<Config::Any|Config::Any>).  If you are using a version of 
Catalyst::Devel prior to 1.06, you can convert to the newer format by 
simply creating the C<myapp.conf> file manually and deleting 
C<myapp.yml>.  The default contents of the C<myapp.conf> you create 
should only consist of one line: 

    name MyApp

B<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);'

=item *

L<Catalyst::Plugin::Static::Simple|Catalyst::Plugin::Static::Simple>

C<Static::Simple> provides an easy way to serve static content, such
as images and CSS files, from the development server.

=back

For our application, we want to add one new plugin into the mix.  To 
do this, edit C<lib/MyApp.pm> (this file is generally referred to as 
your I<application class>) and delete the lines with:

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

Then replace it with:

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

B<Note:> Recent versions of C<Catalyst::Devel> have used a variety of 
techniques to load these plugins/flags.  For example, you might see
the following:

    __PACKAGE__->setup(qw/-Debug ConfigLoader Static::Simple/);

Don't let these variations confuse you -- they all accomplish the same 
result.

This tells Catalyst to start using one additional plugin, 
L<Catalyst::Plugin::StackTrace|Catalyst::Plugin::StackTrace>, to add a 
stack trace to the standard Catalyst "debug screen" (the screen 
Catalyst sends to your browser when an error occurs). Be aware that 
L<StackTrace|Catalyst::Plugin::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.

Make sure when adding new plugins you also include them as a new
dependency within the Makefile.PL file. For example, after adding
the StackTrace plugin the Makefile.PL should include the following
line:

    requires 'Catalyst::Plugin::StackTrace';


B<Notes:> 

=over 4

=item *

C<__PACKAGE__> is just a shorthand way of referencing the name of the 
package where it is used.  Therefore, in C<MyApp.pm>, C<__PACKAGE__> 
is equivalent to C<MyApp>.

=item *

You will want to disable L<StackTrace|Catalyst::Plugin::StackTrace> 
before you put your application into production, but it can be helpful 
during development.

=item *

When specifying plugins, you can omit C<Catalyst::Plugin::> from the 
name.  Additionally, you can spread the plugin names across multiple 
lines as shown here or place them all on one line.

=back


=head1 CREATE A CATALYST CONTROLLER

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

Use the Catalyst C<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 C<lib/MyApp/Controller/Books.pm> (as discussed in Chapter 2 of
the Tutorial, Catalyst has a separate directory under C<lib/MyApp> for
each of the three parts of MVC: C<Model>, C<View>, and C<Controller>)
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::Book')->all]);
        # But, for now, use this code until we create the model later
        $c->stash(books => '');
    
        # 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');
    }

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

Programmers experienced with object-oriented Perl should recognize 
C<$self> as a reference to the object where this method was called. 
On the other hand, C<$c> will be new to many Perl programmers who have 
not used Catalyst before (it's sometimes written as C<$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.

Catalyst actions are regular Perl methods, but they make use of 
attributes (the "C<:Local>" next to the "C<sub list>" in the code 
above) to provide additional information to the Catalyst dispatcher 
logic (note that the space between the colon and the attribute name is 
optional; you will see attributes written both ways).  Most Catalyst 
Controllers use one of five action types:

=over 4

=item *

B<:Private> -- Use C<:Private> for methods that you want to make into 
an action, but you do not want Catalyst to directly expose  the method 
to your users.  Catalyst will not map C<:Private> methods to a URI. 
Use them for various sorts of "special" methods (the C<begin>, 
C<auto>, etc. discussed below) or for methods you want to be able to 
C<forward> or C<detach> to.  (If the method is a plain old "helper 
method" that you don't want to be an action at all, then just define 
the method without any attribute -- you can call it in your code, but 
the Catalyst dispatcher will ignore it.)

There are five types of "special" built-in C<:Private> actions:
C<begin>, C<end>, C<default>, C<index>, and C<auto>.

=over 4

=item *

With C<begin>, C<end>, C<default>, C<index> private actions, only the
most specific action of each type will be called.  For example, if you
define a C<begin> action in your controller it will I<override> a
C<begin> action in your application/root controller -- I<only> the
action in your controller will be called.

=item *

Unlike the other actions where only a single method is called for each
request, I<every> auto action along the chain of namespaces will be
called.  Each C<auto> action will be called I<from the application/root
controller down through the most specific class>.

=back

=item *

B<:Path> -- C<:Path> actions let you map a method to an explicit URI 
path.  For example, "C<:Path('list')>" in 
C<lib/MyApp/Controller/Books.pm> would match on the URL 
C<http://localhost:3000/books/list>, but "C<:Path('/list')>" would 
match on C<http://localhost:3000/list> (because of the leading slash). 
You can use C<:Args()> to specify how many arguments an action should 
accept.  See L<Catalyst::Manual::Intro/Action_types> for more 
information and examples.

=item *

B<:Local> -- C<:Local> is merely a shorthand for 
"C<:Path('_name_of_method_')>".  For example, these are equivalent:
"C<sub create_book :Local {...}>" and 
"C<sub create_book :Path('create_book') {...}>".

=item *

B<:Global> -- C<:Global> is merely a shorthand for 
"C<:Path('/_name_of_method_')>".  For example, these are equivalent:
"C<sub create_book :Global {...}>" and 
"C<sub create_book :Path('/create_book') {...}>".

=item *

B<:Chained> -- Newer Catalyst applications tend to use the Chained 
dispatch form of action types because of its power and flexibility.  
It allows a series of controller methods to be automatically dispatched
to service a single user request.  See 
L<Catalyst::Manual::Tutorial::04_BasicCRUD|Catalyst::Manual::Tutorial::04_BasicCRUD> 
and L<Catalyst::DispatchType::Chained|Catalyst::DispatchType::Chained> 
for more information on chained actions.

=back

You should refer to L<Catalyst::Manual::Intro/Action_types> for 
additional information and for coverage of some lesser-used action 
types not discussed here (C<Regex> and C<LocalRegex>).


=head1 CATALYST VIEWS

As mentioned in Chapter 2 of the tutorial, views are where you render
output, typically for display in the user's web browser (but also
possibly using into output-generation systems, such as PDF or JSON).
The code in C<lib/MyApp/View> selects the I<type> of view to use, with
the actual rendering template found in the C<root> directory.  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 L<http://www.template-toolkit.org>). Other
somewhat popular view technologies include Mason
(L<http://www.masonhq.com> and L<http://www.masonbook.com>) and
L<HTML::Template> (L<http://html-template.sourceforge.net>).


=head2 Create a Catalyst View

When using TT for the Catalyst view, the main helper script
is L<Catalyst::Helper::View::TT|Catalyst::Helper::View::TT>.
You may also come across references to 
L<Catalyst::Helper::View::TTSite|Catalyst::Helper::View::TTSite>,
but its use is now deprecated.

Enter the following command to enable the C<TT> style of view
rendering for this tutorial:

    $ script/myapp_create.pl view HTML TT
     exists "/home/me/MyApp/script/../lib/MyApp/View"
     exists "/home/me/MyApp/script/../t"
     created "/home/me/MyApp/script/../lib/MyApp/View/HTML.pm"
     created "/home/me/MyApp/script/../t/view_HTML.t"

This simply creates a view called C<HTML> in a file called C<HTML.pm> (the first
argument). It is now up to you to decide how you want to structure your view
layout.  For the tutorial, we will start with a very simple TT template to
initially demonstrate the concepts, but quickly migrate to a more typical
"wrapper page" type of configuration (where the "wrapper" controls the overall
"look and feel" of your site from a single file or set of files).

Edit C<lib/MyApp/View/HTML.pm> and you should see
something similar to the following:

    __PACKAGE__->config(
        TEMPLATE_EXTENSION => '.tt',
        render_die => 1,
    );

And update it to match:

    __PACKAGE__->config(
        # Change default TT extension
        TEMPLATE_EXTENSION => '.tt2',
        render_die => 1,
    );

This changes the default extension for Template Toolkit from '.tt' to
'.tt2'.

You can also configure components in your application class. For example,
Edit C<lib/MyApp.pm> and you should see that the default:

    __PACKAGE__->setup;

Above this, add config:

    __PACKAGE__->config(
        'View::HTML' => {
            #Set the location for TT files
            INCLUDE_PATH => [
                __PACKAGE__->path_to( 'root', 'src' ),
            ],
        },
    );
    # This line was here already
    __PACKAGE__->setup;

This changes the base directory for your template files from
C<root> to C<root/src>.

The reason to do this outside the C<lib/MyApp/View/HTML.pm> file
is that the template path is found with the C<path_to> method,
to get a path relative to the application root (no matter where it
is installed), but this requires the application to be loaded...

Trying to set this setting in the view means that you have a chicken
and egg problem, in that the view requires the application to be loaded,
but loading the application loads the view.

Putting the configuration which depends on the application class into
that class is the neatest way to avoid this issue.

Please stick with the settings above for the duration of the
tutorial, but feel free to use whatever options you desire in your
applications (as with most things Perl, there's more than one way to
do it...).

B<Note:> We will use C<root/src> as the base directory for our 
template files, with a full naming convention of 
C<root/src/_controller_name_/_action_name_.tt2>.  Another popular option is to
use C<root/> as the base (with a full filename pattern of 
C<root/_controller_name_/_action_name_.tt2>).

=head2 Create a TT Template Page

First create a directory for book-related TT templates:

    $ mkdir -p root/src/books

Then create C<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 -%]
    [% 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>
        <td></td>
      </tr>
    [% END -%]
    </table>

As indicated by the inline comments above, the C<META title> line uses
TT's META feature to provide a title to the "wrapper" that we will
create later. Meanwhile, the C<FOREACH> loop iterates through each
C<book> model object and prints the C<title> and C<rating> fields.

The C<[%> and C<%]> tags are used to delimit Template Toolkit 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
L<http://search.cpan.org/perldoc?Template::Manual::Variables> for
details and examples).  In addition to the usual L<Template> module Pod
documentation, you can access the TT manual at
L<http://search.cpan.org/perldoc?Template::Manual>.

B<TIP:> While you can build all sorts of complex logic into your TT 
templates, you should in general keep the "code" part of your 
templates as simple as possible.  If you need more complex logic, 
create helper methods in your model that abstract out a set of code 
into a single call from your TT template.  (Note that the same is true 
of your controller logic as well -- complex sections of code in your 
controllers should often be pulled out and placed into your model 
objects.)  In Chapter 4 of the tutorial we will explore some extremely 
helpful and powerful features of L<DBIx::Class> that allow you to pull 
code out of your views and controllers and place it where it 
rightfully belongs in a model class.


=head2 Test Run The Application

To test your work so far, first start the development server:

    $ script/myapp_server.pl -r

Then point your browser to L<http://localhost:3000> and you should
still get the Catalyst welcome page.  Next, change the URL in your
browser to L<http://localhost:3000/books/list>.  If you have
everything working so far, you should see a web page that displays
nothing other than our column headers for "Title", "Rating", and
"Author(s)" -- we will not see any books until we get the database and
model working below.

If you run into problems getting your application to run correctly, it
might be helpful to refer to some of the debugging techniques covered in
the L<Debugging|Catalyst::Manual::Tutorial::07_Debugging> chapter of the
tutorial.


=head1 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.  We will use 
SQLite (L<http://www.sqlite.org>), a popular database that is
lightweight and easy to use. Be sure to get at least version 3. Open
C<myapp01.sql> in your editor and enter:

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

Then use the following command to build a C<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 C<rm myapp.db> command to delete the database before you use
the C<sqlite3 myapp.db E<lt> myapp01.sql> command.

Once the C<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 3.6.22
    Enter ".help" for instructions
    Enter SQL statements terminated with a ";"
    sqlite> select * from book;
    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 book"
    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.

Please note that here we have chosen to use 'singular' table names. This is
because the default inflection code for older versions of
L<DBIx::Class::Schema::Loader> does NOT handle plurals. There has been much
philosophical discussion on whether table names should be plural or singular.
There is no one correct answer, as long as one makes a choice and remains
consistent with it. If you prefer plural table names (e.g. you think that they
are easier to read) then see the documentation in
L<DBIx::Class::Schema::Loader::Base/naming> (version 0.05 or greater).

For using other databases, such as PostgreSQL or MySQL, see 
L<Appendix 2|Catalyst::Manual::Tutorial::10_Appendices>.


=head1 DATABASE ACCESS WITH DBIx::Class

Catalyst can be used with virtually any form of datastore available 
via Perl.  For example, L<Catalyst::Model::DBI|Catalyst::Model::DBI> 
can be used to access databases through the traditional Perl L<DBI> 
interface or you can use a model to access files of any type on the 
filesystem.  However, most Catalyst applications use some form of 
object-relational mapping (ORM) technology to create objects 
associated with tables in a relational database.  Matt Trout's 
L<DBIx::Class|DBIx::Class> (abbreviated as "DBIC") has rapidly emerged 
as the Perl-based ORM technology of choice. Most new Catalyst 
applications rely on DBIx::Class, as will this tutorial.

Although DBIx::Class has included support for a C<create=dynamic> mode 
to automatically read the database structure every time the 
application starts, its use is no longer recommended.  While it can
make for "flashy" demos, the use of the C<create=static> mode we use 
below can be implemented just as quickly and provides many advantages 
(such as the ability to add your own methods to the overall DBIC 
framework, a technique that we see in Chapter 4).


=head2 Make Sure You Have a Recent Version of the DBIx::Class Model

First, let's be sure we have a recent version of the DBIC helper,
L<Catalyst::Model::DBIC::Schema|Catalyst::Model::DBIC::Schema>, so
that we can take advantage of some recent enhancements in how
foreign keys are handled with SQLite.  To check your version, 
run this command:

    $ perl -MCatalyst::Model::DBIC::Schema -e \
        'print "$Catalyst::Model::DBIC::Schema::VERSION\n"'
    0.4

Please note the '\' above.  Depending on your environment, you might 
be able to cut and paste the text as shown or need to remove the '\' 
character to that the command is all on a single line.

If you are following along in Debian 5, you should have version 0.40 or 
higher (shown above as "0.4" with the tailing zero removed). If you have 
less than v0.39, you will need to run this command to install it 
directly from CPAN: 

    $ sudo cpan Catalyst::Model::DBIC::Schema

And re-run the version print command to verify that you are now at 
0.39 or higher.

In addition, since we are using SQLite's foreign key support here,
please be sure that you use version C<1.27> of L<DBD::SQLite> or later:

    $ perl -MDBD::SQLite -e 'print "$DBD::SQLite::VERSION\n"'
    1.29

Upgrade if you are not at version C<1.27> or higher.


=head2 Create Static DBIx::Class Schema Files

Before you continue, make sure your C<myapp.db> database file is in 
the application's topmost directory. Now use the model helper with 
the C<create=static> option to read the database with 
L<DBIx::Class::Schema::Loader|DBIx::Class::Schema::Loader> and 
automatically build the required files for us:

    $ script/myapp_create.pl model DB DBIC::Schema MyApp::Schema \
        create=static dbi:SQLite:myapp.db \
        on_connect_do="PRAGMA foreign_keys = ON"
     exists "/home/me/MyApp/script/../lib/MyApp/Model"
     exists "/home/me/MyApp/script/../t"
    Dumping manual schema for MyApp::Schema to directory /home/me/MyApp/script/../lib ...
    Schema dump completed.
    created "/home/me/MyApp/script/../lib/MyApp/Model/DB.pm"
    created "/home/me/MyApp/script/../t/model_DB.t"

Please note the '\' above.  Depending on your environment, you might 
be able to cut and paste the text as shown or need to remove the '\' 
character to that the command is all on a single line.

The C<script/myapp_create.pl> command breaks down like this:

=over 4

=item *

C<DB> is the name of the model class to be created by the helper in 
C<lib/MyApp/Model>.

=item *

C<DBIC::Schema> is the type of the model to create.

=item *

C<MyApp::Schema> is the name of the DBIC schema file written to
C<lib/MyApp/Schema.pm>.

=item *

C<create=static> causes 
L<DBIx::Class::Schema::Loader|DBIx::Class::Schema::Loader> to 
load the schema as it runs and then write that information out
into files.

=item *

C<dbi:SQLite:myapp.db> is the standard DBI connect string 
for use with SQLite.

=item *

And finally, the C<on_connect_do> string requests that 
L<DBIx::Class::Schema::Loader|DBIx::Class::Schema::Loader> create 
foreign key relationships for us (this is not needed for databases such 
as PostgreSQL and MySQL, but is required for SQLite). If you take a look 
at C<lib/MyApp/Model/DB.pm>, you will see that the SQLite pragma is 
propogated to the Model, so that SQLite's recent (and optional) foreign 
key enforcement is enabled at the start of every database connection. 



=back

If you look in the C<lib/MyApp/Schema.pm> file, you will find that it 
only contains a call to the C<load_namespaces> method.  You will also 
find that C<lib/MyApp> contains a C<Schema> subdirectory, which then 
has a subdirectory called "Result".  This "Result" subdirectory then 
has files named according to each of the tables in our simple database 
(C<Author.pm>, C<BookAuthor.pm>, and C<Book.pm>).  These three 
files are called "Result Classes" in DBIx::Class nomenclature. Although the 
Result Class files are named after tables in our database, the classes 
correspond to the I<row-level data> that is returned by DBIC (more on 
this later, especially in 
L<Catalyst::Manual::Tutorial::04_BasicCRUD/EXPLORING THE POWER OF DBIC>).

The idea with the Result Source files created under 
C<lib/MyApp/Schema/Result> by the C<create=static> option is to only 
edit the files below the C<# 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 automatically created 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 
C<lib/MyApp/Model/DB.pm>.  This file contains a reference to 
C<lib/MyApp/Schema.pm>, so that file is loaded next.  Finally, the 
call to C<load_namespaces> in C<Schema.pm> will load each of the 
"Result Class" files from the C<lib/MyApp/Schema/Result> subdirectory. 
The final outcome is that Catalyst will dynamically 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).

B<NOTE:> Older versions of 
L<Catalyst::Model::DBIC::Schema|Catalyst::Model::DBIC::Schema> use the 
deprecated DBIx::Class C<load_classes> technique instead of the newer 
C<load_namespaces>.  For new applications, please try to use 
C<load_namespaces> since it more easily supports a very useful DBIC 
technique called "ResultSet Classes."  If you need to convert an 
existing application from "load_classes" to "load_namespaces," you can 
use this process to automate the migration, but first make sure you have
version C<0.39> of L<Catalyst::Model::DBIC::Schema> and
L<DBIx::Class::Schema::Loader> version C<0.05000> or later.

    $ # Re-run the helper to upgrade for you
    $ script/myapp_create.pl model DB DBIC::Schema MyApp::Schema \
        create=static naming=current use_namespaces=1 \
        dbi:SQLite:myapp.db \
        on_connect_do="PRAGMA foreign_keys = ON"


=head1 ENABLE THE MODEL IN THE CONTROLLER

Open C<lib/MyApp/Controller/Books.pm> and un-comment the model code we 
left disabled earlier so that your version matches the following (un-
comment the line containing C<[$c-E<gt>model('DB::Book')-E<gt>all]> 
and delete the next 2 lines):

    =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::Book')->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');
    }

B<TIP>: You may see the C<$c-E<gt>model('DB::Book')> un-commented 
above written as C<$c-E<gt>model('DB')-E<gt>resultset('Book')>.  The 
two are equivalent.  Either way, C<$c-E<gt>model> returns a 
L<DBIx::Class::ResultSet|DBIx::Class::ResultSet> which handles queries 
against the database and iterating over the set of results that is 
returned.

We are using the C<-E<gt>all> to fetch all of the books.  DBIC 
supports a wide variety of more advanced operations to easily do 
things like filtering and sorting the results.  For example, the 
following could be used to sort the results by descending title:

    $c->model('DB::Book')->search({}, {order_by => 'title DESC'});

Some other examples are provided in 
L<DBIx::Class::Manual::Cookbook/Complex WHERE clauses>, with 
additional information found at L<DBIx::Class::ResultSet/search>, 
L<DBIx::Class::Manual::FAQ/Searching>, 
L<DBIx::Class::Manual::Intro|DBIx::Class::Manual::Intro> 
and L<Catalyst::Model::DBIC::Schema|Catalyst::Model::DBIC::Schema>.


=head2 Test Run The Application

First, let's enable an environment variable that causes DBIx::Class to 
dump the SQL statements used to access the database.  This is a 
helpful trick when you are trying to debug your database-oriented 
code.  Press C<Ctrl-C> to break out of the development server and
enter:

    $ export DBIC_TRACE=1
    $ script/myapp_server.pl -r 

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

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

Then launch the Catalyst development server.  The log output should
display something like:

    $ script/myapp_server.pl -r
    [debug] Debug messages enabled
    [debug] Statistics enabled
    [debug] Loaded plugins:
    .----------------------------------------------------------------------------.
    | Catalyst::Plugin::ConfigLoader  0.27                                       |
    | Catalyst::Plugin::StackTrace  0.11                                         |
    '----------------------------------------------------------------------------'
    
    [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::Author                                        | class    |
    | MyApp::Model::DB::Book                                          | class    |
    | MyApp::Model::DB::BookAuthor                                    | class    |
    | MyApp::View::HTML                                               | instance |
    '-----------------------------------------------------------------+----------'
    
    [debug] Loaded Private actions:
    .----------------------+--------------------------------------+--------------.
    | Private              | Class                                | Method       |
    +----------------------+--------------------------------------+--------------+
    | /default             | MyApp::Controller::Root              | default      |
    | /end                 | MyApp::Controller::Root              | end          |
    | /index               | MyApp::Controller::Root              | index        |
    | /books/index         | MyApp::Controller::Books             | index        |
    | /books/list          | MyApp::Controller::Books             | list         |
    '----------------------+--------------------------------------+--------------'
    
    [debug] Loaded Path actions:
    .-------------------------------------+--------------------------------------.
    | Path                                | Private                              |
    +-------------------------------------+--------------------------------------+
    | /                                   | /default                             |
    | /                                   | /index                               |
    | /books                              | /books/index                         |
    | /books/list                         | /books/list                          |
    '-------------------------------------+--------------------------------------'
    
    [info] MyApp powered by Catalyst 5.80020
    You can connect to your server at http://debian:3000

B<NOTE:> Be sure you run the C<script/myapp_server.pl> command from
the 'base' directory of your application, not inside the C<script>
directory itself or it will not be able to locate the C<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:

=over 4

=item *

Catalyst::Model::DBIC::Schema dynamically created three model classes,
one to represent each of the three tables in our database
(C<MyApp::Model::DB::Author>, C<MyApp::Model::DB::BookAuthor>,
and C<MyApp::Model::DB::Book>).

=item *

The "list" action in our Books controller showed up with a path of
C</books/list>.

=back

Point your browser to L<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
L<http://localhost:3000/books/list>. You should get a list of the five
books loaded by the C<myapp01.sql> script above without any formatting.
The rating for each book should appear on each row, but the "Author(s)"
column will still be blank (we will fill that in later).

Also notice in the output of the C<script/myapp_server.pl> that 
DBIx::Class used the following SQL to retrieve the data:

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

because we enabled DBIC_TRACE.

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


=head1 CREATE A WRAPPER FOR THE VIEW

When using TT, you can (and should) create a wrapper that will
literally wrap content around each of your templates.  This is
certainly useful as you have one main source for changing things that
will appear across your entire site/application instead of having to
edit many individual files.


=head2 Configure HTML.pm For The Wrapper

In order to create a wrapper, you must first edit your TT view and
tell it where to find your wrapper file. 

Edit your TT view in C<lib/MyApp/View/HTML.pm> and change it to match the 
following:

    __PACKAGE__->config(
        # Change default TT extension
        TEMPLATE_EXTENSION => '.tt2',
        # Set the location for TT files
        INCLUDE_PATH => [
                MyApp->path_to( 'root', 'src' ),
            ],
        # Set to 1 for detailed timer stats in your HTML as comments
        TIMER              => 0,
        # This is your wrapper template located in the 'root/src'
        WRAPPER => 'wrapper.tt2',
    );


=head2 Create the Wrapper Template File and Stylesheet

Next you need to set up your wrapper template.  Basically, you'll want
to take the overall layout of your site and put it into this file.
For the tutorial, open C<root/src/wrapper.tt2> and input the following:

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
    <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
    <head>
    <title>[% template.title or "My Catalyst App!" %]</title>
    <link rel="stylesheet" href="[% c.uri_for('/static/css/main.css') %]" />
    </head>
    
    <body>
    <div id="outer">
    <div id="header">
        [%# Your logo could go here -%]
        <img src="[% c.uri_for('/static/images/btn_88x31_powered.png') %]" />
        [%# Insert the page title -%]
        <h1>[% template.title or site.title %]</h1>
    </div>
    
    <div id="bodyblock">
    <div id="menu">
        Navigation:
        <ul>
            <li><a href="[% c.uri_for('/books/list') %]">Home</a></li>
            <li><a href="[% c.uri_for('/') %]" title="Catalyst Welcome Page">Welcome</a></li>
        </ul>
    </div><!-- end menu -->
    
    <div id="content">
        [%# Status and error messages %]
        <span class="message">[% status_msg %]</span>
        <span class="error">[% error_msg %]</span>
        [%# This is where TT will stick all of your template's contents. -%]
        [% content %]
    </div><!-- end content -->
    </div><!-- end bodyblock -->
    
    <div id="footer">Copyright (c) your name goes here</div>
    </div><!-- end outer -->
    
    </body>
    </html>

Notice the status and error message sections in the code above:

    <span class="status">[% status_msg %]</span>
    <span class="error">[% error_msg %]</span>

If we set either message in the Catalyst stash (e.g.,
C<$c-E<gt>stash-E<gt>{status_msg} = 'Request was successful!'>) it
will be displayed whenever any view used by that request is rendered.
The C<message> and C<error> CSS styles can be customized to suit your
needs in the C<root/static/css/main.css> file we create below.

B<Notes:> 

=over 4

=item *

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

=item *

Although it is beyond the scope of this tutorial, you may wish to use
a JavaScript or AJAX tool such as jQuery (L<http://www.jquery.com>) or
Dojo (L<http://www.dojotoolkit.org>).

=back


=head3 Create A Basic Stylesheet

First create a central location for stylesheets under the static
directory:

    $ mkdir root/static/css

Then open the file C<root/static/css/main.css> (the file referenced in
the stylesheet href link of our wrapper above) and add the following
content:

    #header {
        text-align: center;
    }
    #header h1 {
        margin: 0;
    }
    #header img {
        float: right;
    }
    #footer {
        text-align: center;
        font-style: italic;
        padding-top: 20px;
    }
    #menu {
        font-weight: bold;
        background-color: #ddd;
    }
    #menu ul {
        list-style: none;
        float: left;
        margin: 0;
        padding: 0 0 50% 5px;
        font-weight: normal;
        background-color: #ddd;
        width: 100px;
    }
    #content {
        margin-left: 120px;
    }
    .message {
        color: #390;
    }
    .error {
        color: #f00;
    }

You may wish to check out a "CSS Framework" like Emastic
(L<http://code.google.com/p/emastic/>) as a way to quickly
provide lots of high-quality CSS functionality.


=head2 Test Run The Application

Hit "Reload" in your web browser and you should now see a formatted 
version of our basic book list. (Again, the development server should 
have automatically restarted when you made changes to 
C<lib/MyApp/View/HTML.pm>. If you are not using the "-r" option, you will 
need to hit C<Ctrl-C> and manually restart it. Also note that the 
development server does I<NOT> need to restart for changes to the TT and 
static files we created and edited in the C<root> directory -- those 
updates are handled on a per-request basis.) 

Although our wrapper and stylesheet are obviously very simple, you 
should see how it allows us to control the overall look of an entire 
website from two central files. To add new pages to the site, just 
provide a template that fills in the C<content> section of our wrapper 
template -- the wrapper will provide the overall feel of the page. 


=head2 Updating the Generated DBIx::Class Result Class Files

If you take a look at the Schema files automatically generated by 
L<DBIx::Class::Schema::Loader>, you will see that it has already defined 
C<has_many> and C<belongs_to> relationships on each side of our foreign 
keys. For example, take a look at C<lib/MyApp/Schema/Result/Book.pm> and 
notice the following code: 

    =head1 RELATIONS

    =head2 book_authors

    Type: has_many

    Related object: L<MyApp::Schema::Result::BookAuthor>

    =cut

    __PACKAGE__->has_many(
      "book_authors",
      "MyApp::Schema::Result::BookAuthor",
      { "foreign.book_id" => "self.id" },
    );

Each C<Book> "has_many" C<book_authors>, where C<BookAuthor> is
the many-to-many table that allows each Book to have multiple
Authors, and each Author to have mulitple books.  The arguments
to C<has_many> are:

=over 4

=item *

C<book_authors> - The name for this relationship.  DBIC will create
an accessor on the C<Books> DBIC Row object with this name.

=item *

C<MyApp::Schema::Result::BookAuthor> - The name of the DBIC model
class referenced by this C<has_many> relationship.

=item *

C<foreign.book_id> - C<book_id> is the name of the foreign key 
column in the I<foreign> table that points back to this table.

=item *

C<self.id> - C<id> is the name of the column in I<this> table
that is referenced by the foreign key.

=back

See L<DBIx::Class::Relationship/has_many> for
additional information.  Note that you might see a "hand coded"
version of the C<has_many> relationship above expressed as:

    __PACKAGE__->has_many(
      "book_authors",
      "MyApp::Schema::Result::BookAuthor",
      "book_id",
    );

Where the third argument is simply the name of the column in
the foreign table.  However, the hashref syntax used by 
L<DBIx::Class::Schema::Loader> is more flexible (for example,
it can handle "multi-column" foreign keys).

B<Note:> If you are using older versions of SQLite and related DBIC 
tools, you will need to manually define your C<has_many> and 
C<belongs_to> relationships. We recommend upgrading to the versions 
specified above. :-) 

Have a look at C<lib/MyApp/Schema/Result/BookAuthor.pm> and notice
that there is a C<belongs_to> relationship defined that acts as the
"mirror image" to the C<has_many> relationship we just looked at
above:

    =head1 RELATIONS

    =head2 book

    Type: belongs_to

    Related object: L<MyApp::Schema::Result::Book>

    =cut

    __PACKAGE__->belongs_to(
      "book",
      "MyApp::Schema::Result::Book",
      { id => "book_id" },
      { join_type => "LEFT" },
    );

The arguments are similar, but see 
L<DBIx::Class::Relationship/belongs_to> for the details.

Although recent versions of SQLite and L<DBIx::Class::Schema::Loader> 
automatically handle the C<has_many> and C<belongs_to> relationships, 
C<many_to_many> relationship bridges (not technically a relationship)
currently need to be manually inserted. 
To add a C<many_to_many> relationship bridge, first edit 
C<lib/MyApp/Schema/Result/Book.pm> and add the following text below 
the C<# You can replace this text...> comment:

    # many_to_many():
    #   args:
    #     1) Name of relationship bridge, 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');

B<Note:> Be careful to put this code I<above> the C<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 C<true>.  This is customarily done with
C<1;> on a line by itself.

The C<many_to_many> relationship bridge is optional, but it makes it
easier to map a book to its collection of authors.  Without 
it, we would have to "walk" through the C<book_author> table as in 
C<$book-E<gt>book_author-E<gt>first-E<gt>author-E<gt>last_name> (we 
will see examples on how to use DBIx::Class objects in your code soon, 
but note that because C<$book-E<gt>book_author> can return multiple 
authors, we have to use C<first> to display a single author). 
C<many_to_many> allows us to use the shorter 
C<$book-E<gt>author-E<gt>first-E<gt>last_name>. Note that you cannot 
define a C<many_to_many> relationship bridge without also having the 
C<has_many> relationship in place.

Then edit C<lib/MyApp/Schema/Result/Author.pm> and add the reverse 
C<many_to_many> relationship bridge for C<Author> as follows (again, be careful 
to put in above the C<1;> but below the C<# DO NOT MODIFY THIS OR 
ANYTHING ABOVE!> comment): 

    # many_to_many():
    #   args:
    #     1) Name of relationship bridge, 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_authors', 'book');


=head2 Run The Application

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

    $ DBIC_TRACE=1 script/myapp_server.pl -r

Make sure that the application loads correctly and that you see the
three dynamically created model class (one for each of the
Result Classes we created).

Then hit the URL L<http://localhost:3000/books/list> with your browser 
and be sure that the book list still displays correctly.

B<Note:> You will not see the authors yet because the view does not yet 
use the new relations. Read on to the next section where we update the 
template to do that.


=head1 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 C<root/src/books/list.tt2> and replace
the "empty" table cell "<td></td>" with the following:

    ...
    <td>
      [% # NOTE: See Chapter 4 for a better way to do this!                      -%]
      [% # 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 doesn't return -%]
      [% # a value, so nothing will be printed here.  But, if you have something -%]
      [% # in TT that does return a value and you don't want it printed, you     -%]
      [% # 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>
    ...

B<IMPORTANT NOTE:> Again, you should keep as much "logic code" as 
possible out of your views.  This kind of logic belongs in your model 
(the same goes for controllers -- keep them as "thin" as possible and 
push all of the "complicated code" out to your model objects).  Avoid 
code like you see in the previous example -- we are only using it here 
to show some extra features in TT until we get to the more advanced 
model features we will see in Chapter 4 (see 
L<Catalyst::Manual::Tutorial::04_BasicCRUD/EXPLORING THE POWER OF DBIC>).

Then hit "Reload" in your browser (note that you don't need to reload 
the development server or use the C<-r> option when updating TT 
templates) and you should now see the number of authors each book has 
along with a comma-separated list of the authors' last names.  (If you 
didn't leave the development server running from the previous step, 
you will obviously need to start it before you can refresh your 
browser window.)

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

    SELECT me.id, me.title, me.rating FROM book me:
    SELECT author.id, author.first_name, author.last_name FROM book_author me  
    JOIN author author ON author.id = me.author_id WHERE ( me.book_id = ? ): '1'
    SELECT author.id, author.first_name, author.last_name FROM book_author me  
    JOIN author author ON author.id = me.author_id WHERE ( me.book_id = ? ): '2'
    SELECT author.id, author.first_name, author.last_name FROM book_author me  
    JOIN author author ON author.id = me.author_id WHERE ( me.book_id = ? ): '3'
    SELECT author.id, author.first_name, author.last_name FROM book_author me  
    JOIN author author ON author.id = me.author_id WHERE ( me.book_id = ? ): '4'
    SELECT author.id, author.first_name, author.last_name FROM book_author me  
    JOIN author author ON author.id = me.author_id WHERE ( me.book_id = ? ): '5'

Also note in C<root/src/books/list.tt2> that we are using "| html", a 
type of TT filter, to escape characters such as E<lt> and E<gt> to &lt; 
and &gt; 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 
L<Template::Filters|Template::Filters>.


=head1 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 C<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
L<http://localhost:3000/books/list> with the normal development server
and asked your browser to view the page source.


=head1 OPTIONAL INFORMATION

B<NOTE: The rest of this chapter of the tutorial is optional.  You can
skip to Chapter 4, L<Basic CRUD|Catalyst::Manual::Tutorial::04_BasicCRUD>,
if you wish.>


=head2 Using 'RenderView' for the Default View

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
L<Catalyst::Action::RenderView|Catalyst::Action::RenderView> by
default to automatically perform this operation.  If you look in
C<lib/MyApp/Controller/Root.pm>, you should see the empty
definition for the C<sub end> method:

    sub end : ActionClass('RenderView') {}

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

=over 4

=item *

C<Root.pm> is designed to hold application-wide logic.

=item *

At the end of a given user request, Catalyst will call the most specific
C<end> method that's appropriate.  For example, if the controller for a
request has an C<end> method defined, it will be called.  However, if
the controller does not define a controller-specific C<end> method, the
"global" C<end> method in C<Root.pm> will be called.

=item *

Because the definition includes an C<ActionClass> attribute, the
L<Catalyst::Action::RenderView|Catalyst::Action::RenderView> logic
will be executed B<after> any code inside the definition of C<sub end>
is run.  See L<Catalyst::Manual::Actions|Catalyst::Manual::Actions>
for more information on C<ActionClass>.

=item *

Because C<sub end> is empty, this effectively just runs the default
logic in C<RenderView>.  However, you can easily extend the
C<RenderView> logic by adding your own code inside the empty method body
(C<{}>) created by the Catalyst Helpers when we first ran the
C<catalyst.pl> to initialize our application.  See
L<Catalyst::Action::RenderView|Catalyst::Action::RenderView> for more
detailed information on how to extend C<RenderView> in C<sub end>.

=back


=head2 RenderView's "dump_info" Feature

One of the nice features of C<RenderView> is that it automatically 
allows you to add C<dump_info=1> to the end of any URL for your 
application and it will force the display of the "exception dump" 
screen to the client browser.  You can try this out by pointing 
your browser to this URL: 

    http://localhost:3000/books/list?dump_info=1

You should get a page with the following message at the top:

    Caught exception in MyApp::Controller::Root->end "Forced debug - 
    Scrubbed output at /usr/share/perl5/Catalyst/Action/RenderView.pm line 46."

Along with a summary of your application's state at the end of the 
processing for that request.  The "Stash" section should show a 
summarized version of the DBIC book model objects.  If desired, you 
can adjust the summarization logic (called "scrubbing" logic) -- see 
L<Catalyst::Action::RenderView|Catalyst::Action::RenderView> for 
details.

Note that you shouldn't need to worry about "normal clients" using 
this technique to "reverse engineer" your application -- C<RenderView> 
only supports the C<dump_info=1> feature when your application is 
running in C<-Debug> mode (something you won't do once you have your 
application deployed in production).


=head2 Using The Default Template Name

By default, C<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<$c-E<gt>stash-E<gt>{template} = 'books/list.tt2';> line of our
C<list> action in the Books controller.  Open
C<lib/MyApp/Controller/Books.pm> in your editor and comment out this line
to match the following (only the C<$c-E<gt>stash-E<gt>{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::Book')->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');
    }


You should now be able to access the L<http://localhost:3000/books/list>
URL as before.

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

B<IMPORTANT:> Make sure that you do NOT skip the following section
before continuing to the next chapter 4 Basic CRUD.


=head2 Return To A Manually Specified Template

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

    $c->stash(template => 'books/list.tt2');

Then delete the C<TEMPLATE_EXTENSION> line in
C<lib/MyApp/View/HTML.pm>.

Check the L<http://localhost:3000/books/list> URL in your browser.
It should look the same manner as with earlier sections.


=head1 AUTHOR

Kennedy Clark, C<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
L<http://dev.catalyst.perl.org/repos/Catalyst/Catalyst-Manual/5.80/trunk/lib/Catalyst/Manual/Tutorial/>.

Copyright 2006-2010, Kennedy Clark, under the
Creative Commons Attribution Share-Alike License Version 3.0
(L<http://creativecommons.org/licenses/by-sa/3.0/us/>).