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Kennedy Clark
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NAME

Catalyst::Manual::Tutorial::BasicCRUD - Catalyst Tutorial - Part 4: Basic CRUD

OVERVIEW

This is Part 4 of 10 for the Catalyst tutorial.

Tutorial Overview

  1. Introduction

  2. Catalyst Basics

  3. More Catalyst Basics

  4. Basic CRUD

  5. Authentication

  6. Authorization

  7. Debugging

  8. Testing

  9. Advanced CRUD

  10. Appendices

DESCRIPTION

This part of the tutorial builds on the fairly primitive application created in Part 3 to add basic support for Create, Read, Update, and Delete (CRUD) of Book objects. Note that the 'list' function in Part 2 already implements the Read portion of CRUD (although Read normally refers to reading a single object; you could implement full read functionality using the techniques introduced below). This section will focus on the Create and Delete aspects of CRUD. More advanced capabilities, including full Update functionality, will be addressed in Part 9.

Although this part of the tutorial will show you how to build CRUD functionality yourself, another option is to use a "CRUD builder" type of tool to automate the process. You get less control, but it's quick and easy. For example, see CatalystX::ListFramework::Builder, CatalystX::CRUD, and CatalystX::CRUD::YUI.

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

FORMLESS SUBMISSION

Our initial attempt at object creation will utilize the "URL arguments" feature of Catalyst (we will employ the more common form- based submission in the sections that follow).

Include a Create Action in the Books Controller

Edit lib/MyApp/Controller/Books.pm and enter the following method:

    =head2 url_create
    
    Create a book with the supplied title, rating, and author
    
    =cut
    
    sub url_create : Local {
        # In addition to self & context, get the title, rating, & 
        # author_id args from the URL.  Note that Catalyst automatically 
        # puts extra information after the "/<controller_name>/<action_name/" 
        # into @_
        my ($self, $c, $title, $rating, $author_id) = @_;
    
        # Call create() on the book model object. Pass the table 
        # columns/field values we want to set as hash values
        my $book = $c->model('DB::Books')->create({
                title  => $title,
                rating => $rating
            });
        
        # Add a record to the join table for this book, mapping to 
        # appropriate author
        $book->add_to_book_authors({author_id => $author_id});
        # Note: Above is a shortcut for this:
        # $book->create_related('book_authors', {author_id => $author_id});
        
        # Assign the Book object to the stash for display in the view
        $c->stash->{book} = $book;
    
        # This is a hack to disable XSUB processing in Data::Dumper
        # (it's used in the view).  This is a work-around for a bug in
        # the interaction of some versions or Perl, Data::Dumper & DBIC.
        # You won't need this if you aren't using Data::Dumper (or if
        # you are running DBIC 0.06001 or greater), but adding it doesn't 
        # hurt anything either.
        $Data::Dumper::Useperl = 1;
    
        # Set the TT template to use
        $c->stash->{template} = 'books/create_done.tt2';
    }

Notice that Catalyst takes "extra slash-separated information" from the URL and passes it as arguments in @_. The url_create action then uses a simple call to the DBIC create method to add the requested information to the database (with a separate call to add_to_book_authors to update the join table). As do virtually all controller methods (at least the ones that directly handle user input), it then sets the template that should handle this request.

Include a Template for the url_create Action:

Edit root/src/books/create_done.tt2 and then enter:

    [% # Use the TT Dumper plugin to Data::Dumper variables to the browser   -%]
    [% # Not a good idea for production use, though. :-)  'Indent=1' is      -%]
    [% # optional, but prevents "massive indenting" of deeply nested objects -%]
    [% USE Dumper(Indent=1) -%]
    
    [% # Set the page title.  META can 'go back' and set values in templates -%]
    [% # that have been processed 'before' this template (here it's for      -%]
    [% # root/lib/site/html and root/lib/site/header).  Note that META on    -%]
    [% # simple strings (e.g., no variable interpolation).                   -%]
    [% META title = 'Book Created' %]
    
    [% # Output information about the record that was added.  First title.       -%]
    <p>Added book '[% book.title %]'
    
    [% # Output the last name of the first author.  This is complicated by an    -%]
    [% # issue in TT 2.15 where blessed hash objects are not handled right.      -%]
    [% # First, fetch 'book.authors' from the DB once.                           -%]
    [% authors = book.authors %]
    [% # Now use IF statements to test if 'authors.first' is "working". If so,   -%]
    [% # we use it.  Otherwise we use a hack that seems to keep TT 2.15 happy.   -%]
    by '[% authors.first.last_name IF authors.first; 
           authors.list.first.value.last_name IF ! authors.first %]'
    
    [% # Output the rating for the book that was added -%]
    with a rating of [% book.rating %].</p>
    
    [% # Provide a link back to the list page                                    -%]
    [% # 'uri_for()' builds a full URI; e.g., 'http://localhost:3000/books/list' -%]
    <p><a href="[% c.uri_for('/books/list') %]">Return to list</a></p>
    
    [% # Try out the TT Dumper (for development only!) -%]
    <pre>
    Dump of the 'book' variable:
    [% Dumper.dump(book) %]
    </pre>

The TT USE directive allows access to a variety of plugin modules (TT plugins, that is, not Catalyst plugins) to add extra functionality to the base TT capabilities. Here, the plugin allows Data::Dumper "pretty printing" of objects and variables. Other than that, the rest of the code should be familiar from the examples in Part 3.

Try the url_create Feature

If the application is still running from before, use Ctrl-C to kill it. Then restart the server:

    $ DBIC_TRACE=1 script/myapp_server.pl

Note that new path for /books/url_create appears in the startup debug output.

TIP: You can use script/myapp_server.pl -r to have the development server auto-detect changed files and reload itself (if your browser acts odd, you should also try throwing in a -k). If you make changes to the TT templates only, you do not need to reload the development server (only changes to "compiled code" such as Controller and Model .pm files require a reload).

Next, use your browser to enter the following URL:

    http://localhost:3000/books/url_create/TCPIP_Illustrated_Vol-2/5/4

Your browser should display "Added book 'TCPIP_Illustrated_Vol-2' by 'Stevens' with a rating of 5." along with a dump of the new book model object as it was returned by DBIC. You should also see the following DBIC debug messages displayed in the development server log messages if you have DBIC_TRACE set:

    INSERT INTO books (rating, title) VALUES (?, ?): `5', `TCPIP_Illustrated_Vol-2'
    INSERT INTO book_authors (author_id, book_id) VALUES (?, ?): `4', `6'
    SELECT author.id, author.first_name, author.last_name 
        FROM book_authors me  JOIN authors author 
        ON ( author.id = me.author_id ) WHERE ( me.book_id = ? ): '6'

The INSERT statements are obviously adding the book and linking it to the existing record for Richard Stevens. The SELECT statement results from DBIC automatically fetching the book for the Dumper.dump(book).

If you then click the "Return to list" link, you should find that there are now six books shown (if necessary, Shift+Reload or Ctrl+Reload your browser at the /books/list page).

CONVERT TO A CHAINED ACTION

Although the example above uses the same Local action type for the method that we saw in the previous part of the tutorial, there is an alternate approach that allows us to be more specific while also paving the way for more advanced capabilities. Change the method declaration for url_create in lib/MyApp/Controller/Books.pm you entered above to match the following:

    sub url_create :Chained('/') :PathPart('books/url_create') :Args(3) {

This converts the method to take advantage of the Chained action/dispatch type. Chaining let's you have a single URL automatically dispatch to several controller methods, each of which can have precise control over the number of arguments that it will receive. A chain can essentially be thought of having three parts -- a beginning, a middle and an end. The bullets below summarize the key points behind each of these parts of a chain:

  • Beginning

    • Use ":Chained('/')" to start a chain

    • Get arguments through CaptureArgs()

    • Specify the path to match with PathPart()

  • Middle

    • Link to previous part of the chain with :Chained('_name_')

    • Get arguments through CaptureArgs()

    • Specify the path to match with PathPart()

  • End

    • Link to previous part of the chain with :Chained('_name_')

    • Do NOT get arguments through "CaptureArgs()," use "Args()" instead to end a chain

    • Specify the path to match with PathPart()

In our url_create method above, we have combined all 3 parts into a single method: :Chained('/') to start the chain, :PathPart('books/url_create') to specify the base URL to match, along with :Args(3) to capture exactly 3 arguments and also end the chain.

As we will see shortly, a chain can consist of as many "links" as you wish, with each part capturing some arguments and doing some work along the way. We will continue to use the Chained action type in this part of the tutorial and explore slightly more advanced capabilities with the base method and delete feature below. But Chained dispatch is capable of far more. For additional information, see "Action types" in Catalyst::Manual::Intro, Catalyst::DispatchType::Chained, and the 2006 advent calendar entry on the subject: http://www.catalystframework.org/calendar/2006/10.

Try the Chained Action

If you look back at the development server startup logs from your initial version of the url_create method (the one using the :Local attribute), you will notice that it produced output similar to the following:

    [debug] Loaded Path actions:
    .-------------------------------------+--------------------------------------.
    | Path                                | Private                              |
    +-------------------------------------+--------------------------------------+
    | /                                   | /default                             |
    | /                                   | /index                               |
    | /books                              | /books/index                         |
    | /books/list                         | /books/list                          |
    | /books/url_create                   | /books/url_create                    |
    '-------------------------------------+--------------------------------------'

Now start the development server with our basic chained method in place and the startup debug output should change to something along the lines of the following:

    [debug] Loaded Path actions:
    .-------------------------------------+--------------------------------------.
    | Path                                | Private                              |
    +-------------------------------------+--------------------------------------+
    | /                                   | /default                             |
    | /                                   | /index                               |
    | /books                              | /books/index                         |
    | /books/list                         | /books/list                          |
    '-------------------------------------+--------------------------------------'
    
    [debug] Loaded Chained actions:
    .-------------------------------------+--------------------------------------.
    | Path Spec                           | Private                              |
    +-------------------------------------+--------------------------------------+
    | /books/url_create/*/*/*             | /books/url_create                    |
    '-------------------------------------+--------------------------------------'

url_create has disappeared form the "Loaded Path actions" section but it now shows up under the newly created "Loaded Chained actions" section. And, the "/*/*/*" portion clearly shows our requirement for three arguments.

As with our non-chained version of url_create, use your browser to enter the following URL:

    http://localhost:3000/books/url_create/TCPIP_Illustrated_Vol-2/5/4

You should see the same "Added book 'TCPIP_Illustrated_Vol-2' by 'Stevens' with a rating of 5." along with a dump of the new book model object. Click the "Return to list" link, you should find that there are now seven books shown (two copies of TCPIP_Illustrated_Vol-2).

Refactor to Use a "Base" Method to Start the Chains

Let's make a quick update to our initial Chained action to show a little more of the power of chaining. First, open lib/MyApp/Controller/Books.pm in your editor and add the following method:

    =head2 base
    
    Can place common logic to start chained dispatch here
    
    =cut
    
    sub base :Chained('/') :PathPart('books') :CaptureArgs(0) {
        my ($self, $c) = @_;
        
        # Store the resultset in stash so it's available for other methods
        $c->stash->{resultset} = $c->model('DB::Books');
    
        # Print a message to the debug log
        $c->log->debug('*** INSIDE BASE METHOD ***');
    }

Here we print a log message and store the DBIC resultset in $c->stash->{resultset} so that it's automatically available for other actions that chain off base. If your controller always needs a book ID as it's first argument, you could have the base method capture that argument (with :CaptureArgs(1)) and use it to pull the book object with ->find($id) and leave it in the stash for later parts of your chains to then act upon. Because we have several actions that don't need to retrieve a book (such as the url_create we are working with now), we will instead add that functionality to a common object action shortly.

As for url_create, let's modify it to first dispatch to base. Open up lib/MyApp/Controller/Books.pm and edit the declaration for url_create to match the following:

    sub url_create :Chained('base') :PathPart('url_create') :Args(3) {

Next, try out the refactored chain by restarting the development server. Notice that our "Loaded Chained actions" section has changed slightly:

    [debug] Loaded Chained actions:
    .-------------------------------------+--------------------------------------.
    | Path Spec                           | Private                              |
    +-------------------------------------+--------------------------------------+
    | /books/url_create/*/*/*             | /books/base (0)                      |
    |                                     | => /books/url_create                 |
    '-------------------------------------+--------------------------------------'

The "Path Spec" is the same, but now it maps to two Private actions as we would expect.

Once again, enter the following URL into your browser:

    http://localhost:3000/books/url_create/TCPIP_Illustrated_Vol-2/5/4

The same "Added book 'TCPIP_Illustrated_Vol-2' by 'Stevens' with a rating of 5" message and dump of the new book object should appear. Also notice the extra debug message in the development server output from the base method. Click the "Return to list" link, you should find that there are now eight books shown.

MANUALLY BUILDING A CREATE FORM

Although the url_create action in the previous step does begin to reveal the power and flexibility of both Catalyst and DBIC, it's obviously not a very realistic example of how users should be expected to enter data. This section begins to address that concern.

Add Method to Display The Form

Edit lib/MyApp/Controller/Books.pm and add the following method:

    =head2 form_create
    
    Display form to collect information for book to create
    
    =cut
    
    sub form_create :Chained('base') :PathPart('form_create') :Args(0) {
        my ($self, $c) = @_;
    
        # Set the TT template to use
        $c->stash->{template} = 'books/form_create.tt2';
    }

This action simply invokes a view containing a book creation form.

Add a Template for the Form

Open root/src/books/form_create.tt2 in your editor and enter:

    [% META title = 'Manual Form Book Create' -%]
    
    <form method="post" action="[% c.uri_for('form_create_do') %]">
    <table>
      <tr><td>Title:</td><td><input type="text" name="title"></td></tr>
      <tr><td>Rating:</td><td><input type="text" name="rating"></td></tr>
      <tr><td>Author ID:</td><td><input type="text" name="author_id"></td></tr>
    </table>
    <input type="submit" name="Submit" value="Submit">
    </form>

Note that we have specified the target of the form data as form_create_do, the method created in the section that follows.

Add a Method to Process Form Values and Update Database

Edit lib/MyApp/Controller/Books.pm and add the following method to save the form information to the database:

    =head2 form_create_do
    
    Take information from form and add to database
    
    =cut
    
    sub form_create_do :Chained('base') :PathPart('form_create_do') :Args(0) {
        my ($self, $c) = @_;
    
        # Retrieve the values from the form
        my $title     = $c->request->params->{title}     || 'N/A';
        my $rating    = $c->request->params->{rating}    || 'N/A';
        my $author_id = $c->request->params->{author_id} || '1';
    
        # Create the book
        my $book = $c->model('DB::Books')->create({
                title   => $title,
                rating  => $rating,
            });
        # Handle relationship with author
        $book->add_to_book_authors({author_id => $author_id});
    
        # Store new model object in stash
        $c->stash->{book} = $book;
    
        # Avoid Data::Dumper issue mentioned earlier
        # You can probably omit this    
        $Data::Dumper::Useperl = 1;
    
        # Set the TT template to use
        $c->stash->{template} = 'books/create_done.tt2';
    }

Test Out The Form

If the application is still running from before, use Ctrl-C to kill it. Then restart the server:

    $ script/myapp_server.pl

Notice that the server startup log reflects the two new chained methods that we added:

    [debug] Loaded Chained actions:
    .-------------------------------------+--------------------------------------.
    | Path Spec                           | Private                              |
    +-------------------------------------+--------------------------------------+
    | /books/form_create                  | /books/base (0)                      |
    |                                     | => /books/form_create                |
    | /books/form_create_do               | /books/base (0)                      |
    |                                     | => /books/form_create_do             |
    | /books/url_create/*/*/*             | /books/base (0)                      |
    |                                     | => /books/url_create                 |
    '-------------------------------------+--------------------------------------'

Point your browser to http://localhost:3000/books/form_create and enter "TCP/IP Illustrated, Vol 3" for the title, a rating of 5, and an author ID of 4. You should then see the output of the same create_done.tt2 template seen in earlier examples. Finally, click "Return to list" to view the full list of books.

Note: Having the user enter the primary key ID for the author is obviously crude; we will address this concern with a drop-down list in Part 9.

A SIMPLE DELETE FEATURE

Turning our attention to the delete portion of CRUD, this section illustrates some basic techniques that can be used to remove information from the database.

Edit root/src/books/list.tt2 and update it to the following (two sections have changed: 1) the additional '<th>Links</th>' table header, and 2) the four lines for the Delete link near the bottom).

    [% # 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><th>Links</th></tr>
    [% # Display each book in a table row %]
    [% FOREACH book IN books -%]
      <tr>
        <td>[% book.title %]</td>
        <td>[% book.rating %]</td>
        <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   -%]
          ([% tt_authors.size %])
          [% # Use another TT vmethod to join & print the names & comma separators   -%]
          [% tt_authors.join(', ') %]
        </td>
        <td>
          [% # Add a link to delete a book %]
          <a href="[% c.uri_for(c.controller.action_for('delete'), [book.id]) %]">Delete</a>
        </td>
      </tr>
    [% END -%]
    </table>

The additional code is obviously designed to add a new column to the right side of the table with a Delete "button" (for simplicity, links will be used instead of full HTML buttons).

Also notice that we are using a more advanced form of uri_for than we have seen before. Here we use $c->controller- >action_for to automatically generate a URI appropriate for that action based on the method we want to link to while inserting the book.id value into the appropriate place. Now, if you ever change :PathPart('delete') in your controller method to :PathPart('kill'), then your links will automatically update without any changes to your .tt2 template file. As long as the name of your method does not changed ("delete" here), then your links will still be correct. There are a few shortcuts and options when using action_for():

  • If you are referring to a method in the current controller, you can use $self->action_for('_method_name_').

  • If you are referring to a method in a different controller, you need to include that controller's name as an argument to controller(), as in $c->controller('_controller_name_')->action_for('_method_name_').

Note: In general, you should use more than just a simple link with your applications. Consider using some sort of of confirmation page (typically with unique actions in your controller for both the confirmation and the actual delete operation). Also, you should try to use an HTTP POST operation (versus the GET used here) for operations that change the state of your application (e.g., the database).

Add a Common Method to Retrieve a Book for the Chain

As mentioned earlier, since we have a mixture of actions that operate on a single book ID and others that do no, we should not have base capture the book ID, find the corresponding book in the database and save it in the stash for later links in the chain. However, just because that logic does not belong in base doesn't mean that we can't create another location to centralize the book lookup code. In our case, we will create a method called object that will store the specific book in the stash. Chains that always operate on a single existing book can chain off this method, but methods such as url_create that don't operate on an existing book can chain directly off base.

To add the object method, edit lib/MyApp/Controller/Books.pm and add the following code:

    =head2 object
    
    Fetch the specified book object based on the book ID and store
    it in the stash
    
    =cut
    
    sub object :Chained('base') :PathPart('id') :CaptureArgs(1) {
        # $id = primary key of book to delete
        my ($self, $c, $id) = @_;
        
        # Find the book object and store it in the stash
        $c->stash(object => $c->stash->{resultset}->find($id));
        
        # Make sure the lookup was successful.  You would probably
        # want to do something like this in a real app:
        #   $c->detach('/error_404') if !$c->stash->{object};
        die "Book $id not found!" if !$c->stash->{object};
    }

Now, any other method that chains off object will automatically have the appropriate book waiting for it in $c->stash-Egt{object}>.

Also note that we are using different technique for setting $c->stash. The advantage of this style is that it let's you set multiple stash variables at a time. For example:

    $c->stash(object => $c->stash->{resultset}->find($id),
              another_thing => 1);

or as a hashref:

    $c->stash({object => $c->stash->{resultset}->find($id),
              another_thing => 1});

Either format works, but the $c->stash(name => value); style is growing in popularity -- you may which to use it all the time (even when you are only setting a single value).

Add a Delete Action to the Controller

Open lib/MyApp/Controller/Books.pm in your editor and add the following method:

    =head2 delete
    
    Delete a book
        
    =cut
    
    sub delete :Chained('object') :PathPart('delete') :Args(0) {
        my ($self, $c) = @_;
    
        # Use the book object saved by 'object' and delete it along
        # with related 'book_authors' entries
        $c->stash->{object}->delete;
    
        # Set a status message to be displayed at the top of the view
        $c->stash->{status_msg} = "Book deleted.";
    
        # Forward to the list action/method in this controller
        $c->forward('list');
    }

This method first deletes the book object saved by the object method. However, it also removes the corresponding entry from the book_authors table with a cascading delete.

Then, rather than forwarding to a "delete done" page as we did with the earlier create example, it simply sets the status_msg to display a notification to the user as the normal list view is rendered.

The delete action uses the context forward method to return the user to the book list. The detach method could have also been used. Whereas forward returns to the original action once it is completed, detach does not return. Other than that, the two are equivalent.

Try the Delete Feature

If the application is still running from before, use Ctrl-C to kill it. Then restart the server:

    $ DBIC_TRACE=1 script/myapp_server.pl

The delete method now appears in the "Loaded Chained actions" section of the startup debug output:

    [debug] Loaded Chained actions:
    .-------------------------------------+--------------------------------------.
    | Path Spec                           | Private                              |
    +-------------------------------------+--------------------------------------+
    | /books/id/*/delete                  | /books/base (0)                      |
    |                                     | -> /books/object (1)                 |
    |                                     | => /books/delete                     |
    | /books/form_create                  | /books/base (0)                      |
    |                                     | => /books/form_create                |
    | /books/form_create_do               | /books/base (0)                      |
    |                                     | => /books/form_create_do             |
    | /books/url_create/*/*/*             | /books/base (0)                      |
    |                                     | => /books/url_create                 |
    '-------------------------------------+--------------------------------------'

Then point your browser to http://localhost:3000/books/list and click the "Delete" link next to the first "TCPIP_Illustrated_Vol-2". A green "Book deleted" status message should display at the top of the page, along with a list of the eight remaining books. You will also see the cascading delete operation via the DBIC_TRACE output:

    DELETE FROM books WHERE ( id = ? ): '6'
    SELECT me.book_id, me.author_id FROM book_authors me WHERE ( me.book_id = ? ): '6'
    DELETE FROM book_authors WHERE ( author_id = ? AND book_id = ? ): '4', '6'

Fixing a Dangerous URL

Note the URL in your browser once you have performed the deletion in the prior step -- it is still referencing the delete action:

    http://localhost:3000/books/delete/6

What if the user were to press reload with this URL still active? In this case the redundant delete is harmless (although it does generate an exception screen, it doesn't perform any undesirable actions on the application or database), but in other cases this could clearly be extremely dangerous.

We can improve the logic by converting to a redirect. Unlike $c->forward('list')) or $c->detach('list')) that perform a server-side alteration in the flow of processing, a redirect is a client-side mechanism that causes the browser to issue an entirely new request. As a result, the URL in the browser is updated to match the destination of the redirection URL.

To convert the forward used in the previous section to a redirect, open lib/MyApp/Controller/Books.pm and edit the existing sub delete method to match:

    =head2 delete
    
    Delete a book
    
    =cut
    
    sub delete :Chained('object') :PathPart('delete') :Args(0) {
        my ($self, $c) = @_;
    
        # Use the book object saved by 'object' and delete it along
        # with related 'book_authors' entries
        $c->stash->{object}->delete;
    
        # Set a status message to be displayed at the top of the view
        $c->stash->{status_msg} = "Book deleted.";
    
        # Redirect the user back to the list page.  Note the use
        # of $self->action_for as earlier in this section (BasicCRUD)
        $c->response->redirect($c->uri_for($self->action_for('list')));
    }

Try the Delete and Redirect Logic

Restart the development server and point your browser to http://localhost:3000/books/list (don't just hit "Refresh" in your browser since we left the URL in an invalid state in the previous section!) and delete the first copy of the remaining two "TCPIP_Illustrated_Vol-2" books. The URL in your browser should return to the http://localhost:3000/books/list URL, so that is an improvement, but notice that no green "Book deleted" status message is displayed. Because the stash is reset on every request (and a redirect involves a second request), the status_msg is cleared before it can be displayed.

Using uri_for to Pass Query Parameters

There are several ways to pass information across a redirect. One option is to use the flash technique that we will see in Part 5 of the tutorial; however, here we will pass the information via query parameters on the redirect itself. Open lib/MyApp/Controller/Books.pm and update the existing sub delete method to match the following:

    =head2 delete 
    
    Delete a book
        
    =cut
    
    sub delete :Chained('object') :PathPart('delete') :Args(0) {
        my ($self, $c) = @_;
    
        # Use the book object saved by 'object' and delete it along
        # with related 'book_authors' entries
        $c->stash->{object}->delete;
    
        # Redirect the user back to the list page with status msg as an arg
        $c->response->redirect($c->uri_for($self->action_for('list'), 
            {status_msg => "Book deleted."}));
    }

This modification simply leverages the ability of uri_for to include an arbitrary number of name/value pairs in a hash reference. Next, we need to update root/src/wrapper.tt2 to handle status_msg as a query parameter:

    ...
    <div id="content">
        [%# Status and error messages %]
        <span class="message">[% status_msg || c.request.params.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 -->
    ...

Although the sample above only shows the content div, leave the rest of the file intact -- the only change we made to the wrapper.tt2 was to add "|| c.request.params.status_msg" to the <span class="message"> line.

Try the Delete and Redirect With Query Param Logic

Restart the development server and point your browser to http://localhost:3000/books/list (you should now be able to safely hit "refresh" in your browser). Then delete the remaining copy of "TCPIP_Illustrated_Vol-2". The green "Book deleted" status message should return.

NOTE: Another popular method for maintaining server-side information across a redirect is to use the flash technique we discuss in the next part of the tutorial, Authentication. While flash is a "slicker" mechanism in that it's all handled by the server and doesn't "pollute" your URLs, it is important to note that flash can lead to situations where the wrong information shows up in the wrong browser window if the user has multiple windows or browser tabs open. For example, Window A causes something to be placed in the stash, but before that window performs a redirect, Window B makes a request to the server and gets the status information that should really go to Window A. For this reason, you may wish to use the "query param" technique shown here in your applications.

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/Catalyst-Manual/5.70/trunk/lib/Catalyst/Manual/Tutorial/.

Copyright 2006-2008, Kennedy Clark, under Creative Commons License (http://creativecommons.org/licenses/by-sa/3.0/us/).