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Stevan Little > Moose-1.00 > Moose::Cookbook::Meta::Recipe5


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Source   Latest Release: Moose-2.0204


Moose::Cookbook::Meta::Recipe5 - The "table" attribute as a metaclass trait


  package MyApp::Meta::Class::Trait::HasTable;
  use Moose::Role;

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

  package Moose::Meta::Class::Custom::Trait::HasTable;
  sub register_implementation { 'MyApp::Meta::Class::Trait::HasTable' }

  package MyApp::User;
  use Moose -traits => 'HasTable';



This recipe takes the metaclass table attribute from Moose::Cookbook::Meta::Recipe4 and implements it as a metaclass trait. Traits are just roles, as we saw in Moose::Cookbook::Meta::Recipe3.

The advantage of using traits is that it's easy to combine multiple traits, whereas combining multiple metaclass subclasses requires creating yet another subclass. With traits, Moose takes care of applying them to your metaclass.

Using this Metaclass Trait in Practice

Once this trait has been applied to a metaclass, it looks exactly like the example we saw in Moose::Cookbook::Meta::Recipe4:

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

  # the safe version
  $table = MyApp::User->meta->table
      if MyApp::User->meta->meta->can('does')
         and MyApp::User->meta->meta->does('MyApp::Meta::Class');

The safe version is a little complicated. We have to check that the metaclass object's metaclass has a does method, in which case we can ask if the the metaclass does a given role.

It's simpler to just write:

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

In theory, this is a little less correct, since the metaclass might be getting its table method from a different role. In practice, you are unlikely to encounter this sort of problem.


Moose::Cookbook::Meta::Recipe3 - Labels implemented via attribute traits

Moose::Cookbook::Meta::Recipe4 - Adding a "table" attribute to the metaclass


Dave Rolsky <>


Copyright 2006-2010 by Infinity Interactive, Inc.

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

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