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.IX Title "DBIx::Class::Helper::Row::OnColumnChange 3pm"
.TH DBIx::Class::Helper::Row::OnColumnChange 3pm "2014-09-17" "perl v5.20.1" "User Contributed Perl Documentation"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
DBIx::Class::Helper::Row::OnColumnChange \- Do things when the values of a column change
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& package MyApp::Schema::Result::Account;
\&
\& use parent \*(AqDBIx::Class::Core\*(Aq;
\&
\& _\|_PACKAGE_\|_\->load_components(qw(Helper::Row::OnColumnChange));
\&
\& _\|_PACKAGE_\|_\->table(\*(AqAccount\*(Aq);
\&
\& _\|_PACKAGE_\|_\->add_columns(
\&    id => {
\&       data_type         => \*(Aqinteger\*(Aq,
\&       is_auto_increment => 1,
\&    },
\&    amount => {
\&       data_type          => \*(Aqfloat\*(Aq,
\&       keep_storage_value => 1,
\&    },
\& );
\&
\& _\|_PACKAGE_\|_\->before_column_change(
\&   amount => {
\&      method   => \*(Aqbank_transfer\*(Aq,
\&      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($delta)
\&   }
\& }
\&
\& 1;
.Ve
.PP
or with DBIx::Class::Candy:
.PP
.Vb 1
\& package MyApp::Schema::Result::Account;
\&
\& use DBIx::Class::Candy \-components => [\*(AqHelper::Row::OnColumnChange\*(Aq];
\&
\& table \*(AqAccount\*(Aq;
\&
\& column id => {
\&    data_type         => \*(Aqinteger\*(Aq,
\&    is_auto_increment => 1,
\& };
\&
\& column amount => {
\&    data_type          => \*(Aqfloat\*(Aq,
\&    keep_storage_value => 1,
\& };
\&
\& before_column_change amount => {
\&    method   => \*(Aqbank_transfer\*(Aq,
\&    txn_wrap => 1,
\& };
\&
\& sub bank_transfer {
\&   my ($self, $old_value, $new_value) = @_;
\&
\&   my $delta = abs($old_value \- $new_value);
\&   if ($old_value < $new_value) {
\&      Bank\->subtract($delta)
\&   } else {
\&      Bank\->add($delta)
\&   }
\& }
\&
\& 1;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This module codifies a pattern that I've used in a number of projects, namely
that of doing \fBsomething\fR when a column changes it's value in the database.
It leverages DBIx::Class::Helper::Row::StorageValues for passing in the
\&\f(CW$old_value\fR, which do not have to use.  If you leave the
\&\f(CW\*(C`keep_storage_value\*(C'\fR out of the column definition it will just pass \f(CW\*(C`undef\*(C'\fR
in as the \f(CW$old_value\fR.  Also note the \f(CW\*(C`txn_wrap\*(C'\fR option.  This allows you to
specify that you want the call to \f(CW\*(C`update\*(C'\fR and the call to the method you
requested to be wrapped in a transaction.  If you end up calling more than
one method due to multple column change methods and more than one specify
\&\f(CW\*(C`txn_wrap\*(C'\fR it will still only wrap once.
.PP
I've gone to great lengths to ensure that order is preserved, so \f(CW\*(C`before\*(C'\fR
and \f(CW\*(C`around\*(C'\fR changes are called in order of definition and \f(CW\*(C`after\*(C'\fR changes
are called in reverse order.
.PP
To be clear, the change methods only get called if the value will be changed
after \f(CW\*(C`update\*(C'\fR runs.  It correctly looks at the current value of the column
as well as the arguments passed to \f(CW\*(C`update\*(C'\fR.
.SH "CANDY EXPORTS"
.IX Header "CANDY EXPORTS"
If used in conjunction with DBIx::Class::Candy this component will export:
.IP "before_column_change" 4
.IX Item "before_column_change"
.PD 0
.IP "around_column_change" 4
.IX Item "around_column_change"
.IP "after_column_change" 4
.IX Item "after_column_change"
.PD
.SH "METHODS"
.IX Header "METHODS"
.SS "before_column_change"
.IX Subsection "before_column_change"
.Vb 6
\& _\|_PACKAGE_\|_\->before_column_change(
\&   col_name => {
\&      method   => \*(Aqmethod\*(Aq, # <\-\- anything that can be called as a method
\&      txn_wrap => 1,        # <\-\- true if you want it to be wrapped in a txn
\&   }
\& );
.Ve
.PP
Note: the arguments passed to \f(CW\*(C`method\*(C'\fR will be
\&\f(CW\*(C`$self, $old_value, $new_value\*(C'\fR.
.SS "after_column_change"
.IX Subsection "after_column_change"
.Vb 6
\& _\|_PACKAGE_\|_\->after_column_change(
\&   col_name => {
\&      method   => \*(Aqmethod\*(Aq, # <\-\- anything that can be called as a method
\&      txn_wrap => 1,        # <\-\- true if you want it to be wrapped in a txn
\&   }
\& );
.Ve
.PP
Note: the arguments passed to \f(CW\*(C`method\*(C'\fR will be
\&\f(CW\*(C`$self, $old_value, $new_value\*(C'\fR.
.SS "around_column_change"
.IX Subsection "around_column_change"
.Vb 6
\& _\|_PACKAGE_\|_\->around_column_change(
\&   col_name => {
\&      method   => \*(Aqmethod\*(Aq, # <\-\- anything that can be called as a method
\&      txn_wrap => 1,        # <\-\- true if you want it to be wrapped in a txn
\&   }
\& );
.Ve
.PP
Note: the arguments passed to \f(CW\*(C`method\*(C'\fR will be
\&\f(CW\*(C`$self, $next, $old_value, $new_value\*(C'\fR.
.PP
Around is subtly different than the other two callbacks.  You \fBmust\fR call
\&\f(CW$next\fR in your method or it will not work at all.  A silly example of how
this is done could be:
.PP
.Vb 2
\& sub around_change_name {
\&   my ($self, $next, $old, $new) = @_;
\&
\&   my $govt_records = $self\->govt_records;
\&
\&   $next\->();
\&
\&   $govt_records\->update({ name => $new });
\& }
.Ve
.PP
Note: the above code implies a weird database schema.  I haven't actually seen
a time when I've needed around yet, but it seems like there is a use-case.
.PP
Also Note: you don't get to change the args to \f(CW$next\fR.  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.)
.SH "AUTHOR"
.IX Header "AUTHOR"
Arthur Axel \*(L"fREW\*(R" Schmidt <frioux+cpan@gmail.com>
.SH "COPYRIGHT AND LICENSE"
.IX Header "COPYRIGHT AND LICENSE"
This software is copyright (c) 2014 by Arthur Axel \*(L"fREW\*(R" Schmidt.
.PP
This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.