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John Napiorkowski
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Catalyst::ActionRole::QueryParameter - Dispatch rules using query parameters


    package MyApp::Controller::Foo;

    use Moose;
    use namespace::autoclean;

    BEGIN {
      extends 'Catalyst::Controller::ActionRole';

    ## Add the ActionRole to all the Controller's actions.  You can also
    ## selectively add the ActionRole with the :Does action attribute or in
    ## controller configuration.  See Catalyst::Controller::ActionRole for
    ## more information.

      action_roles => ['QueryParameter'],

    ## Match an incoming request matching "http://myhost/path?page"
    sub paged_results : Path('foo') QueryParam('page') { ... }

    ## Match an incoming request matching "http://myhost/path"
    sub no_paging : Path('foo') QueryParam('!page') { ... }


Let's you require conditions on request query parameters (as you would access via $ctx->request->query_parameters) as part of your dispatch matching. This ActionRole is not intended to be used for general HTML form and parameter processing or validation, for that purpose there are many other options (such as HTML::FormHandler, Data::Manager or <HTML::FormFu>.) What it can be useful for is when you want to delegate work to various Actions inside your Controller based on what the incoming query parameters say.

Generally speaking, it is not great development practice to abuse query parameters this way. However I find there is a limited and controlled subset of use cases where this feature is valuable. As a result, the features of this ActionRole are also limited to simple defined or undefined checking, and basic Perl relational operators.

You can specify multiple QueryParams per Action. If you do have more than one we will try to match Actions that match ALL the given QueryParam attributes.

There's a functioning Catalyst example application in the test directory for your review as well.


The value of the QueryParam attribute allows for condition matching based on query parameter definedness and via Perl relational operators. For example, you can match for a particular value or if a given value is greater than another. This can be useful when you want to perform a different Action when (for example) your user is on page 10 of a search, which might indicate they are not finding what they want and could use some additional help. I also sometimes find that I want special handling of the first page of a search result.

Although you can handle this with conditional logic inside your Action, I find the ability to declare what I want from an Action to be one of the more valuable aspects of Catalyst.

Here are some example QueryParam attributes and the queries they match:

    QueryParam('page')  ## 'page' must exist
    QueryParam('!page')  ## 'page' must NOT exist
    QueryParam('page:==1')  ## 'page' must equal numeric one
    QueryParam('page:>1')  ## 'page' must be great than one
    QueryParam('!page:>1')  ## 'page' must NOT be great than one

Since as I mentioned, it is generally not awesome web development practice to make excessive use of query parameters for mapping your action logic, I have limited the condition matching to basic Perl operators. The general pattern is as follows:


Which can be roughly translated as "A $parameter should match the $condition but we can tack a "!" to the front of the expression to reverse the match. If you don't specify a $condition, the default condition is definedness."

A $condition is basically a Perl relational operator followed by a value. Relation Operators we current support: ==,eq,>,<,!=,<=,>=,gt,ge,lt,le. In addition, we support the regular expression match operator =~. For documentation on Perl Relational Operators see: perldoc perlop. For documentation on Perl Regular Expressions see perldoc perlre.

The condition will be wrapped in an eval and any exceptions generated will be taken to mean the pattern has not matched.


You may prefer to set your Query Parameter requirements via the Catalyst general application configuration, rather than in subroutine attributes. Doing so allows you to use different settings in different environments and it also allows you to use more extended values. Here's an example comparing both approaches

    ## subroutine attribute approach
    sub first_page : Path('foo') QueryParam('page:==1') { ... }

    ## configuration approach
      action => {
        first_page => { Path => 'foo', QueryParam => 'page:==1'},

Since the configuration approach allows richer use of Perl, you can replace the string version of the QueryParam value with the following:

    ## configuration approach, richer Perl data structure
      action => {
        first_page => { Path => 'foo', QueryParam => [['page','==','1']] },
        no_page_query => { Path => 'foo', QueryParam => [['!','page']] },

If you are using the configuration approach, this second option is preferred. Please note that since each attribute or configuration key can have an array of values, if you use the 'rich Perl data structure' approach in your configuration you will need to place the arrayref inside an arrayref as in the example above (that is not a typo!)


When several actions match the path of an incoming request, such as in the following example:

    sub no_query : Path('foo') {
      my ($self, $ctx) = @_;

    sub page : Path('foo') QueryParam('page') {
      my ($self, $ctx) = @_;

Catayst will call the match method on each in turn until it finds one that returns a successful match. This matching process starts from the bottom up (or last to first), which means that you should place your most specific matches at the bottom and your least specific or 'catch all' actions at the top.

HOWEVER, if you are using Chained actions Catalyst::DispatchType::Chained then the order resolution is REVERSED from the above example. In other words we start with the first action and proceed downwards. This means that when you are Chaining, you should place you most specific matches FIRST (nearest the top of the Controller file) and least specific or default actions LAST.

For example:

    sub root : Chained('/') PathPrefix CaptureArgs(0) {}

      sub page_and_row
      : Chained('root') PathPart('') QueryParam('page') QueryParam('row') Args(0)
        my ($self, $ctx) = @_;

      sub page : Chained('root') PathPart('')  QueryParam('page') Args(0)  {
        my ($self, $ctx) = @_;

      sub no_query : Chained('root') PathPart('') Args(0)  {
        my ($self, $ctx) = @_;

The test suite has a working example of this for your review.


John Napiorkowski email:jjnapiork@cpan.org


Catalyst, Catalyst::Controller::ActionRole, Moose.


Copyright 2011, John Napiorkowski email:jjnapiork@cpan.org

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.