sub checkout_total {
   +shift->items->get_column("cost")->sum;
}

##</code><code>##

# Inside My::Schema::Cart or an auxillary class only
# loaded into the space on demand.

sub checkout_total {
   # My::Schema::Cart object.
   my $self = shift;
   # items is a has_many relationship.
   my $items_rs = $self->items; # ->items_rs to avoid contex ambiguity.
   # Get a DBIx::Class::ResultSetColumn object for cost.
   my $cost_col = $items_rs->get_column("cost");
   # Perform SUM.
   my $sum = $cost_col->sum;
   return $sum;
}

##</code><code>##

# This is not exactly how it would be set up for real.
# Load order and @INC stuff requires some tweaks to
# have in one file.
BEGIN {
   package My::Schema::Item;
   use strict;
   use warnings;
   use parent "DBIx::Class::Core";
   __PACKAGE__->table("item");
   __PACKAGE__->add_columns(
        "id", { data_type => "integer", is_auto_increment => 1, is_nullable => 0 },
        "cart", { data_type => "integer", is_nullable => 0 },
        "cost",
       );
   __PACKAGE__->set_primary_key("id");
   # See below.
   # __PACKAGE__->belongs_to(cart => "My::Schema::Cart");

   package My::Schema::Cart;
   use strict;
   use warnings;
   use parent "DBIx::Class::Core";
   __PACKAGE__->table("cart");
   __PACKAGE__->add_columns(
        "id", { data_type => "integer", is_auto_increment => 1, is_nullable => 0 },
       );
   __PACKAGE__->set_primary_key("id");
   __PACKAGE__->has_many( items => "My::Schema::Item",
                          { "foreign.cart" => "self.id" } );

   sub checkout_total {
       +shift->items->get_column("cost")->sum;
   }

   # Would actually be above.
   My::Schema::Item->belongs_to(cart => "My::Schema::Cart");

   # This would really be lib/My/Schema.pm
   package My::Schema;
   use strict;
   use warnings;
   use parent "DBIx::Class::Schema";
   __PACKAGE__->load_classes(qw( Item Cart ));
}

use strict;
use warnings;

my $schema = My::Schema->connect("dbi:SQLite::memory:");
$schema->deploy;

my $cart_id = 1024;
$schema->populate( Cart => [ ["id"],[$cart_id] ] );

my $cost = sub { sprintf "%.2f", rand(100) };
for ( 1 .. 6 )
{
   $schema->populate( Item => [ [qw( id cart cost )],
                                [ $_, $cart_id, $cost->() ] ] );
}

for my $cart ( $schema->resultset("Cart")->all )
{
   printf("cart:%d --> \$%6.2f\n",
          $cart->id,
          $cart->checkout_total);

   for my $item ( $cart->items )
   {
       printf("    item:%d -->  \$%5.2f\n",
              $item->id,
              $item->cost);

   }
}
__DATA__
cart:1024 ---> $222.16
    item:1 -->  $46.46
    item:2 -->  $ 5.56
    item:3 -->  $84.50
    item:4 -->  $ 6.72
    item:5 -->  $15.50
    item:6 -->  $63.42