Moose::Cookbook::Meta::Table_MetaclassTrait

Section: User Contributed Perl Documentation (3)
Updated: 2018-05-16
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NAME

Moose::Cookbook::Meta::Table_MetaclassTrait - Adding a "table" attribute as a metaclass trait

VERSION

version 2.2011

SYNOPSIS

  # in lib/MyApp/Meta/Class/Trait/HasTable.pm
  package MyApp::Meta::Class::Trait::HasTable;
  use Moose::Role;
  Moose::Util::meta_class_alias('HasTable');

  has table => (
      is  => 'rw',
      isa => 'Str',
  );

  # in lib/MyApp/User.pm
  package MyApp::User;
  use Moose -traits => 'HasTable';

  __PACKAGE__->meta->table('User');

DESCRIPTION

In this recipe, we'll create a class metaclass trait which has a ``table'' attribute. This trait is for classes associated with a DBMS table, as one might do for an ORM.

In this example, the table name is just a string, but in a real ORM the table might be an object describing the table.

THE METACLASS TRAIT

This really is as simple as the recipe ``SYNOPSIS'' shows. The trick is getting your classes to use this metaclass, and providing some sort of sugar for declaring the table. This is covered in Moose::Cookbook::Extending::Debugging_BaseClassRole, which shows how to make a module like "Moose.pm" itself, with sugar like "has_table()".

Using this Metaclass Trait in Practice

Accessing this new "table" attribute is quite simple. Given a class named "MyApp::User", we could simply write the following:

  my $table = MyApp::User->meta->table;

As long as "MyApp::User" has arranged to apply the "MyApp::Meta::Class::Trait::HasTable" to its metaclass, this method call just works. If we want to be more careful, we can check that the class metaclass object has a "table" method:

  $table = MyApp::User->meta->table
      if MyApp::User->meta->can('table');

In theory, this is not entirely correct, since the metaclass might be getting its "table" method from a different trait. In practice, you are unlikely to encounter this sort of problem.

RECIPE CAVEAT

This recipe doesn't work when you paste it all into a single file. This is because the "use Moose -traits => 'HasTable';" line ends up being executed before the "table" attribute is defined.

When the two packages are separate files, this just works.

AUTHORS

•: Stevan Little <stevan.little@iinteractive.com>
•: Dave Rolsky <autarch@urth.org>
•: Jesse Luehrs <doy@tozt.net>
•: Shawn M Moore <code@sartak.org>
•: יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
•: Karen Etheridge <ether@cpan.org>
•: Florian Ragwitz <rafl@debian.org>
•: Hans Dieter Pearcey <hdp@weftsoar.net>
•: Chris Prather <chris@prather.org>
•: Matt S Trout <mst@shadowcat.co.uk>

COPYRIGHT AND LICENSE

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.