gistfile1.PL

package MusicPref;
use Moose;
use namespace::autoclean;
has 'name' => ( is => 'ro', isa => 'Str' );

package Character;
use Moose;
use namespace::autoclean;
has 'name'        => ( is => 'ro', isa  => 'Str'          );
has 'music_prefs' => ( is => 'rw', does => 'KiokuDB::Set' );

gistfile2.PL

package MusicPref::Extractor;

use Moose;
use namespace::autoclean;

with qw(
    Search::GIN::Extract
    Search::GIN::Keys::Deep
);

sub extract_values {
    my ( $self, $obj, @args ) = @_;
    ref $obj eq 'Character' or return;

    my $set   = $obj->members || return;
    my @prefs = scalar $set->members > 0              ?
                map { $_->music_prefs } $set->members :
                ();

    $self->process_keys( { music_pref => \@prefs } );
}

gistfile3.PL

use MusicPref;
use Character;
use KiokuDB::Util 'set';

# the music preferences
my $jazz    = MusicPref->new( name => 'Jazz'               );
my $blues   = MusicPref->new( name => 'Blues'              );
my $jingles = MusicPref->new( name => 'Commercial Jingles' );

# the characters themselves
my $lisa = Character->new(
    name        => 'Lisa Simpson',
    music_prefs => set( $jazz, $blues ),
);

my $barney = Character->new(
    name        => 'Barney Gumble',
    music_prefs => set($blues),
);

my $homer = Character->new(
    name        => 'Homer Simpson',
    music_prefs => set($jingles),
);

gistfile4.PL

has '+extra_args' => (
    default => sub { {
        extract => MusicPref::Extractor->new,
        # ...
    } },
);

query.PL

use Search::GIN::Query::Manual;

my $query = Search::GIN::Query::Manual->new(
    values => { music_pref => 'blues' },
);

my @characters = $db->search($query)->all; # Lisa, Barney objects