Mixin::ExtraFields - add extra stashes of data to your objects


version 0.140003


If you use the ExtraFields mixin in your class:

  package Corporate::WorkOrder;

  use Mixin::ExtraFields -fields => {
    id      => 'workorder_id',
    moniker => 'note',
    driver  => { HashGuts => { hash_key => '_notes' } }

...your objects will then have methods for manipulating their extra fields:

  my $workorder = Corporate::WorkOrder->retrieve(1234);

  if ($workorder->note_exists('debug_next')) {
    warn $workorder->note_get('debug_next');

  if ($workorder->note_get('time_bomb')) {
      last_explosion  => time,
      explosion_cause => 'time bomb',


Sometimes your well-defined object needs a way to tack on arbirary extra fields. This might be a set of session-specific ephemeral data, a stash of settings that need to be easy to grow over time, or any sort of name-and-value parameters. Adding more and more methods can be cumbersome, and may not be helpful if the names vary greatly. Accessing an object's guts directly is simple, but is difficult to control when subclassing, and can make altering your object's structure difficult.

Mixin::ExtraFields provides a simple way to add an arbitrary number of stashes for named data. These data can be stored in the object, in a database, or anywhere else. The storage mechanism is abstracted away from the provided interface, so one storage mechanism can be easily swapped for another. Multiple ExtraFields stashes can be mixed into one class, using one or many storage mechanisms.


This library should run on perls released even a long time ago. It should work on any version of perl released in the last five years.

Although it may work on older versions of perl, no guarantee is made that the minimum required version will not be increased. The version may be increased for any reason, and there is no promise that patches will be accepted to lower the minimum required perl.


To create a stash of extra fields, just use Mixin::ExtraFields and import the fields group like this:

  use Mixin::ExtraFields -fields => { driver => 'SomeDriver' };

The only argument required for the group is driver, which names the driver (storage mechanism) to use. For more information, see "Specifying a Driver", below.

Other valid arguments are:

  id - the name of the method to call on objects to get their unique identifier
       default: id; an explicit undef will use each object's reference addr

  moniker - the name to use in forming mixed-in method names
            default: extra

Specifying a Driver

The driver argument can be given as either a driver identifier or a reference to a hash of options. If given as a hash reference, one of the entries in the hash must be class, giving the driver identifier for the driver.

A driver identifier must be either:

  • an object of a class descended from the driver base class

  • a partial class name, to follow the driver base class name

  • a full class name, prepended with +

The driver base class is provided by the "driver_base_class" method. In almost all cases, it will be Mixin::ExtraFields::Driver.


The default implementation of Mixin::ExtraFields provides a number of methods for accessing the extras.

Wherever "extra" appears in the following method names, the moniker argument given to the fields group will be used instead. For example, if the use statement looked like this:

 use Mixin::ExtraFields -fields => { moniker => 'info', driver => 'HashGuts' };

...then a method called exists_info would be generated, rather than exists_extra. The fields group also respects renaming options documented in Sub::Exporter.


  if ($obj->exists_extra($name)) { ... }

This method returns true if there is an entry in the extras for the given name.



  my $value = $obj->get_extra($name);

  my $value_hash = $obj->get_detailed_extra($name);

These methods return the entry for the given name. If none exists, the method returns undef. The detailed version of this method will return a hashref describing all information available about the entry. While this information is driver-specific, it is required to have an entry for the key entry, providing the value that would have been returned by get_extra.



  my %extra = $obj->get_all_extra;

  my %extra_hash = $obj->get_all_detailed_extra;

These methods return a list of name/value pairs. The values are in the same form as those returned by the get-by-name methods, above.


  my @names = $obj->get_all_extra_names;

This method returns the names of all existing extras.


  $obj->set_extra($name => $value);

This method sets the given extra. If no entry existed before, one is created. If one existed for this name, it is replaced.



This method deletes the named entry. After deletion, no entry will exist for that name.



This method deletes all entries for the object.


Mixin::ExtraFields can be subclassed to produce different methods, provide different names, or behave differently in other ways. Subclassing Mixin::ExtraFields can produce many distinct and powerful tools.

None of the generated methods, above, are implemented in Mixin::ExtraFields. The methods below are its actual methods, which work together to build and export the methods that are mixed in. These are the methods you should override when subclassing Mixin::ExtraFields.

For information on writing drivers, see Mixin::ExtraFields::Driver.


This method returns the default moniker. The default default moniker defaults to the default "extra".


This method returns a list of base method names to construct and install. These method names will be transformed into the installed method names via "method_name".

  my @methods = Mixin::ExtraFields->methods;


  my $method_name = Mixin::ExtraFields->method_name($method_base, $moniker);

This method returns the method name that will be installed into the importing class. Its default behavior is to join the method base (which comes from the "methods" method) and the moniker with an underscore, more or less.


This method returns the name of the driver method used to implement the given method name. This is primarily useful in the default implementation of MixinExtraFields, where there is a one-to-one correspondence between installed methods and driver methods.

Changing this method could very easily cause incompatibility with standard driver classes, and should only be done by the wise, brave, or reckless.


  my $sub_href = Mixin::ExtraFields->gen_fields_group($name, \%arg, \%col);

This method is a group generator, as used by Sub::Exporter and described in its documentation. It is the method you are least likely to subclass.


  my $code = Mixin::ExtraFields->build_method($method_name, \%arg);

This routine builds the requested method. It is passed a method name in the form returned by the methods method and a hashref of the following data:

  id_method - the method to call on objects to get their unique id
  driver    - the storage driver
  moniker   - the moniker of the set of extras being built

Note! The values for the above arguments are references to the values you'd expect. That is, if the id method is "foo" you will be given an reference to the string foo. (This reduces the copies of common values that will be enclosed into generated code.)


  my $arg = Mixin::ExtraFields->default_driver_arg;

This method a default value for the driver argument to the fields group generator. By default, this method will croak if called.


  my $driver = Mixin::ExtraFields->build_driver($arg);

This method constructs and returns the driver object to be used by the generated methods. It is passed the driver argument given in the importing code's use statement.


This is the name of the name of the class which drivers are expected to subclass. By default it returns Mixin::ExtraFields::Driver.


  • handle invocants without ids (classes) and drivers that don't need ids


Ricardo Signes <>


  • Ricardo SIGNES <>

  • Ricardo Signes <>


This software is copyright (c) 2022 by Ricardo Signes.

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