DBIx::Class::Helper::Row::OnColumnChange(3)

NAME
DBIx::Class::Helper::Row::OnColumnChange - Do things when the values of a column change

VERSION
version 2.019000

SYNOPSIS
 package MyApp::Schema::Result::Account;

 use parent 'DBIx::Class::Core';

 __PACKAGE__->load_components(qw(Helper::Row::OnColumnChange));

 __PACKAGE__->table('Account');

 __PACKAGE__->add_columns(
    id => {
       data_type         => 'integer',
       is_auto_increment => 1,
    },
    amount => {
       data_type          => 'float',
       keep_storage_value => 1,
    },
 );

 __PACKAGE__->before_column_change(
   amount => {
      method   => 'bank_transfer',
      txn_wrap => 1,
   }
 );

 sub bank_transfer {
   my ($self, $old_value, $new_value) = @_;

   my $delta = abs($old_value - $new_value);
   if ($old_value < $new_value) {
      Bank->subtract($delta)
   } else {
      Bank->add($delta)
   }
 }

 1;

or with DBIx::Class::Candy:

 package MyApp::Schema::Result::Account;

 use DBIx::Class::Candy -components => ['Helper::Row::OnColumnChange'];

 table 'Account';

 column id => {
    data_type         => 'integer',
    is_auto_increment => 1,
 };

 column amount => {
    data_type          => 'float',
    keep_storage_value => 1,
 };

 before_column_change amount => {
    method   => 'bank_transfer',
    txn_wrap => 1,
 };

 sub bank_transfer {
   my ($self, $old_value, $new_value) = @_;

   my $delta = abs($old_value - $new_value);
   if ($old_value < $new_value) {
      Bank->subtract($delta)
   } else {
      Bank->add($delta)
   }
 }

 1;

DESCRIPTION
This module codifies a pattern that I've used in a number of projects, namely that of doing something when a column changes it's value in the database. It leverages DBIx::Class::Helper::Row::StorageValues for passing in the $old_value, which do not have to use. If you leave the "keep_storage_value" out of the column definition it will just pass "undef" in as the $old_value. Also note the "txn_wrap" option. This allows you to specify that you want the call to "update" and the call to the method you requested to be wrapped in a transaction. If you end up calling more than one method due to multple column change methods and more than one specify "txn_wrap" it will still only wrap once.

I've gone to great lengths to ensure that order is preserved, so "before" and "around" changes are called in order of definition and "after" changes are called in reverse order.

To be clear, the change methods only get called if the value will be changed after "update" runs. It correctly looks at the current value of the column as well as the arguments passed to "update".

METHODS

  before_column_change
 __PACKAGE__->before_column_change(
   col_name => {
      method   => 'method', # <-- anything that can be called as a method
      txn_wrap => 1,        # <-- true if you want it to be wrapped in a txn
   }
 );

Note: the arguments passed to "method" will be "$self, $old_value, $new_value".

  after_column_change
 __PACKAGE__->after_column_change(
   col_name => {
      method   => 'method', # <-- anything that can be called as a method
      txn_wrap => 1,        # <-- true if you want it to be wrapped in a txn
   }
 );

Note: the arguments passed to "method" will be "$self, $old_value, $new_value".

  around_column_change
 __PACKAGE__->around_column_change(
   col_name => {
      method   => 'method', # <-- anything that can be called as a method
      txn_wrap => 1,        # <-- true if you want it to be wrapped in a txn
   }
 );

Note: the arguments passed to "method" will be "$self, $next, $old_value, $new_value".

Around is subtly different than the other two callbacks. You must call $next in your method or it will not work at all. A silly example of how this is done could be:

 sub around_change_name {
   my ($self, $next, $old, $new) = @_;

   my $govt_records = $self->govt_records;

   $next->();

   $govt_records->update({ name => $new });
 }

Note: the above code implies a weird database schema. I haven't actually seen a time when I've needed around yet, but it seems like there is a use-case.

Also Note: you don't get to change the args to $next. If you think you should be able to, you probably don't understand what this component is for. That or you know something I don't (equally likely.)

CANDY EXPORTS
If used in conjunction with DBIx::Class::Candy this component will export:

before_column_change
around_column_change
after_column_change

AUTHOR
Arthur Axel "fREW" Schmidt <frioux+cpan@gmail.com>

COPYRIGHT AND LICENSE
This software is copyright (c) 2013 by Arthur Axel "fREW" Schmidt.

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