{

    package Foo;
    use Moose;
    use namespace::autoclean;

    has 'bars' => (
        traits  => ['Array'],
        is      => 'ro',
        isa     => 'ArrayRef[Bar]',
        default => sub { [] },
        handles => {
            push_bars   => 'push',
            all_bars    => 'elements',
        },
        lazy => 1,
    );

    sub BUILD {
        my $self = shift;
        $_->push_foos($self) foreach $self->all_bars;
    }

    __PACKAGE__->meta->make_immutable;

    1;
}

{

    package Bar;
    use Moose;
    use namespace::autoclean;

    has 'foos' => (
        traits  => ['Array'],
        is      => 'ro',
        isa     => 'ArrayRef[Foo]',
        default => sub { [] },
        handles => {
            push_foos   => 'push',
            all_foos    => 'elements',
        },
        lazy => 1,
    );

    __PACKAGE__->meta->make_immutable;

    1;
}