# 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;
}