Trying to understand DBIx::Class::CDBICompat

gistfile1.pl

package Unity::DBI;
use base qw/DBIx::Class::CDBICompat/;

package Unity::Cluster;
use base 'Unity::DBI'; 

 __PACKAGE__->table('u_clusters');
 __PACKAGE__->table_alias('u_clusters');
 __PACKAGE__->sequence('obj_id');
 __PACKAGE__->columns(Primary => qw/id/);
 __PACKAGE__->columns(Others => qw/name description/);

package Unity::Sequence;
use base 'Unity::DBI';

 __PACKAGE__->table('u_sequences');
 __PACKAGE__->table_alias('u_sequences');
 __PACKAGE__->sequence('obj_id');
 __PACKAGE__->columns(Primary   => qw/id/);
 __PACKAGE__->columns(Essential => qw/name accession type /);

package Unity::Clustering;
use base 'Unity::DBI';

__PACKAGE__->table('u_clustering');
__PACKAGE__->columns(Primary => qw/cluster_id sequence_id/);
__PACKAGE__->has_a('cluster_id' => 'Unity::Cluster');
__PACKAGE__->has_a('sequence_id' => 'Unity::Sequence');

# Now that Unity::Cluster, Unity::Sequence and Unity::Clustering have been
# defined, I should be able to define a has_many mapping?
Unity::Sequence->has_many(clusters => [qw(Unity::Clustering cluster_id)]);

Error when trying to run the above:

DBIx::Class::CDBICompat::ColumnCase::has_many(): No such column sequence on foreign class Unity::Clustering (using our class name 'Unity::Sequence' as foreign key) at C2Ssimp.pm line 32

The exception stack (via debugger):
at /disk/software/lib/perl5/site_perl/5.8.7/DBIx/Class/Exception.pm line 72
DBIx::Class::Exception::throw('DBIx::Class::Exception', 'No such column sequence on foreign class Unity::Clustering (u...') called at /disk/software/lib/perl5/site_perl/5.8.7/DBIx/Class/Row.pm line 1449
DBIx::Class::Row::throw_exception('Unity::Sequence', 'No such column sequence on foreign class Unity::Clustering (u...') called at /disk/software/lib/perl5/site_perl/5.8.7/DBIx/Class/Relationship/HasMany.pm line 45
DBIx::Class::Relationship::HasMany::has_many('Unity::Sequence', 'clusters', 'Unity::Clustering', '', 'HASH(0x36a1f80)') called at /disk/software/lib/perl5/site_perl/5.8.7/DBIx/Class/CDBICompat/Relationships.pm line 109
DBIx::Class::CDBICompat::Relationships::has_many('Unity::Sequence', 'clusters', 'ARRAY(0x3a19940)', '') called at /disk/software/lib/perl5/site_perl/5.8.7/DBIx/Class/CDBICompat/ColumnCase.pm line 28
DBIx::Class::CDBICompat::ColumnCase::has_many('Unity::Sequence', 'clusters', 'ARRAY(0x3a19940)') called at C2Ssimp.pm line 32