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Maypole::Manual::StandardTemplates(3) User Contributed Perl Documentation Maypole::Manual::StandardTemplates(3)

Maypole::Manual::StandardTemplates - Maypole's Standard Templates and Actions

As we saw in our Create-Read-Update-Delete (CRUD) example, Maypole does all it can to make your life easier; this inclues providing a set of default actions and factory-supplied templates. These are written in such a generic way, making extensive use of class metadata, that they are more or less applicable to any table or application. However, in order to progress from automatically generated CRUD applications to real customized applications, we need to begin by understanding how these default actions do their stuff, and how the default templates are put together. Once we have an understanding of what Maypole does for us automatically, we can begin to customize and create our own templates and actions.
Although the standard templates can be applied in many situations, they're really provided just as examples, as a starting point to create your own templates to suit your needs. The goal of templating is to keep templates simple so the presentation can be changed easily when you desire. We're not trying to build a single set of reusable templates that cover every possible situation.

Remember that actions are just subroutines in the model classes with an Exported attribute. A simple, uncustomized Maypole model class, such as one of the classes in the beer database application, provides the following default actions - that is, provides access to the following URLs:
This takes the ID of an object in a table, retrieves the object, and presents it to the view template.
This is the same as "view", but uses the edit template to provide a web form to edit the object; it submits to "do_edit".
When called with an ID, the "do_edit" action provides row editing.
When called without an ID, the "do_edit" action provides row creation.
This deletes a row, returning to the "list" page.
This provides a paged list of the table suitable for browsing.
This handles a search query and presents the search results back to the list template. Previously this was called search, but obviously that clashes with a lot of stuff, and that usage is now deprecated.
We'll now look at how these actions are implemented, before moving on to take a detailed look at the templates they drive.
"view" and "edit"
These actions are very simple; their job is to take a row ID, turn it into an object, and hand it to the template to be displayed. However, as taking the first argument and turning it into an object is such a common action, it is handled directly by the model class's "process" method. Similarly, the default template name provided by the "process" method is the name of the action, and so will be "view" or "edit" accordingly.
So the code required to make these two actions work turns out to be:
    sub view :Exported { }
    sub edit :Exported { }
That's right - no code at all. This shows the power of the templating side of the system. If you think about it for a moment, it is natural that these actions should not have any code - after all, we have separated out the concerns of "acting" and displaying. Both of these "actions" are purely concerned with displaying a record, and don't need to do any "acting". Remember that the "edit" method doesn't actually do any editing - this is provided by "do_edit"; it is just another view of the data, albeit one which allows the data to be modified later. These two methods don't need to modify the row in any way, they don't need to do anything clever. They just are.
So why do we need the subroutines at all? If the subroutines did not exist, we would be sent to the "view" and "edit" templates as would be expected, but these templates would not be provided with the right arguments; we need to go through the "process" method in order to turn the URL argument into a row and thence into an object to be fed to the template. By exporting these methods, even though they contain no code themselves, we force Maypole to call "process" and provide the class and object to the templates.
The moral of this story is that if you need to have an action which is purely concerned with display, not acting, but needs to receive an ID and turn it into an object, then create an empty method. For instance, if we want to make an alternate view of a row which only showed the important columns, we might create a method
    sub short_view :Exported {}
This will cause the row to be turned into an object and fed to the "short_view" template, and that template would be responsible for selecting the particular columns to be displayed.
This action, on the other hand, actually has to do something. If it's provided with an ID, this is turned into an object and we're in edit mode, acting upon that object. If not, we're in create mode.
    sub do_edit :Exported {
        my ($self, $r) = @_;
        my $h = CGI::Untaint->new(%{$r->params});
        my ($obj) = @{$r->objects || []};
        if ($obj) {
            # We have something to edit
        } else {
            $obj = $self->create_from_cgi($h);
The "CDBI" model uses the "update_from_cgi" and "create_from_cgi" methods of Class::DBI::FromCGI to turn "POST" parameters into database table data. This in turn uses CGI::Untaint to ensure that the data coming in is suitable for the table. If you're using the default "CDBI" model, then, you're going to need to set up your tables in a way that makes "FromCGI" happy.
The data is untainted, and any errors are collected into a hash which is passed to the template. We also pass back in the parameters, so that the template can re-fill the form fields with the original values. The user is then sent back to the "edit" template.
        if (my %errors = $obj->cgi_update_errors) {
            # Set it up as it was:
            $r->template_args->{cgi_params} = $r->params;
            $r->template_args->{errors} = \%errors;
Otherwise, the user is taken back to viewing the new object:
    } else {
    $r->objects([ $obj ]);
Notice that this does use hard-coded names for the templates to go to next. Feel free to override this in your subclasses:
    sub do_edit :Exported {
        my ($class, $r) = @_;
Digression on "Class::DBI::FromCGI"
"CGI::Untaint" is a mechanism for testing that incoming form data conforms to various properties. For instance, given a "CGI::Untaint" object that encapsulates some "POST" parameters, we can extract an integer like so:
    $h->extract(-as_integer => "score");
This checks that the "score" parameter is an integer, and returns it if it is; if not, "$h->error" will be set to an appropriate error message. Other tests by which you can extract your data are "as_hex" and "as_printable", which tests for a valid hex number and an ordinary printable string respectively; there are other handlers available on CPAN, and you can make your own, as documented in CGI::Untaint.
To tell the "FromCGI" handler what handler to use for each of your columns, you need to use the "untaint_columns" methods in the classes representing your tables. For instance:
        integer => ["score", ... ],
This must be done after the call to "setup" in your handler, because otherwise the model classes won't have been set up to inherit from "Class::DBI::FromCGI".
Remember that if you want to use drop-downs to set the value of related fields, such as the brewery for a beer, you need to untaint these as something acceptable for the primary key of that table:
        integer => ["score", "brewery", "style" ],
This is usually integer, if you're using numeric IDs for your primary key. If not, you probably want "printable", but you probably know what you're doing anyway.
The do_delete method takes a number of arguments and deletes those rows from the database; it then loads up all rows and heads to the list template. You almost certainly want to override this to provide some kind of authentication.
Previously this was called delete, but obviously that clashes with a lot of stuff, and that usage is now deprecated.
Listing, like viewing, is a matter of selecting objects for presentation. This time, instead of a single object specified in the URL, we want, by default, all the records in the table:
    sub list :Exported {
        my ($class, $r) = @_;
        $r->objects([ $self->retrieve_all ])
However, things are slightly complicated by paging and ordering by column; the default implementation also provides a "Class::DBI::Pager" object to the templates and uses that to retrieve the appropriate bit of the data, as specified by the "page" URL query parameter. See the "pager" template below.
Searching also uses paging, and creates a query from the "POST" parameters. It uses the list template to display the objects once they've been selected from the database.

Once these actions have done their work, they hand a set of objects to the templates; if you haven't specified your own custom template globally or for a given class, you'll be using the factory specified template. Let's take a look now at each of these and how they're put together.
The beauty of the factory specified templates is that they make use of the classes' metadata as supplied by the view class. Although you're strongly encouraged to write your own templates, in which you don't need to necessarily be as generic, the factory templates will always do the right thing for any class without further modification, and as such are useful examples of how to build Maypole templates.
There are certain common elements to a template, and these are extracted out. For instance, all the templates call the header template to output a HTML header, and nearly all include the macros template to load up some common template functions. We'll look at these common macros as we come across them.
template view
The edit template is pretty much the same as view, but it uses Maypole::Model::CDBI::AsForm's "to_field" method on each column of an object to return a "HTML::Element" object representing a form element to edit that property. These elements are then rendered to HTML with "as_HTML" or to XHTML with "as_XML". It expects to see a list of editing errors, if any, in the "errors" template variable:
     FOR col = classmetadata.columns;
        NEXT IF col == "id";
        "<B>"; classmetadata.colnames.$col; "</B>";
        ": ";
        IF errors.$col;
            "<FONT COLOR=\"#ff0000\">"; errors.$col; "</FONT>";
Browsing records and search results are both handled by the list template. The "search" template argument is used to distinguish between the two cases:
    [% IF search %]
    <h2> Search results </h2>
    [% ELSE %]
    <h2> Listing of all [% classmetadata.plural %]</h2>
    [% END %]
The pager template controls the list of pages at the bottom (by default) of the list and search views. It expects a "pager" template argument which responds to the Data::Page interface. There's a description of how it works in the Template Toolkit section of the View chapter.
The macros template is included at the start of most other templates and makes some generally-useful template macros available:
"link(table, command, additional, label)"
This makes an HTML link pointing to "/base/table/command/additional" labelled by the text in label. "base" is the template variable that contains the base URL of this application.
"maybe_link_view" takes something returned from the database - either some ordinary data, or an object in a related class expanded by a has-a relationship. If it is an object, it constructs a link to the view command for that object. Otherwise, it just displays the data.
"display_line" is used in the list template to display a row from the database, by iterating over the columns and displaying the data for each column. It misses out the "id" column by default, and magically URLifies columns called "url". This may be considered too much magic for some.
"button(object, action)"
This is a simple button that is submitted to "/base/table/action/id", where "table" and "id" are those belonging to the database row "object". The button is labelled with the name of the action. You can see buttons on many pages, including lists.
This takes an object, and looks up its "related_accessors"; this gives a list of accessor methods that can be called to get a list of related objects. It then displays a title for that accessor, (e.g. "Beers" for a "brewery.beers") calls the accessor, and displays a list of the results. You can see it in use at the bottom of the standard view pages.
Contents, Next The Request Workflow, Previous Maypole View Classes,
2006-10-02 perl v5.28.1

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