The Solution - code

• ScalarAttributeToHasA isn't necessary!

• Subclass Class::DBI::DataMigration::Mapping and override map():

package Legacy::Mapping::GameVendorToPublisher;

use base qw/Class::DBI::DataMigration::Mapping/;
use New::DBI::Publisher;
use Carp;

sub map {
   my ($self, $source_key, $source_object) = @_;

   # We're about to call $source_object->$source_key, 
   # so let's make sure we know how to do that:
   my $source_class = ref $source_object;
   eval "require $source_class" unless $source_class->can('new');
   confess $@ if $@;

   # Now find_or_create the target object:
   return New::DBI::Publisher->
       find_or_create({ name => $source_object->$source_key });
}

• Install somewhere in @INC so that Mapper can find it