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//_related_ids => a:4:{i:0;i:2462;i:1;i:2466;i:2;i:2469;i:3;i:2472;}
//manage_product_posts_custom_column
add_filter('manage_edit-product_columns', 'related_product_col');

function related_product_col($columns) {
    $new_columns = (is_array($columns)) ? $columns : array();
    $new_columns['RELATED'] = 'Related Product';
    return $new_columns;
}

add_action('manage_product_posts_custom_column', 'related_product_col_data', 2);

function related_product_col_data($column) {
    global $post;
    $related_product_ids = get_post_meta($post->ID, '_related_ids', true);
    if ($column == 'RELATED') {
        if (isset($related_product_ids) && !empty($related_product_ids)) {
            echo count($related_product_ids) . ' [' . implode(', ', $related_product_ids) . ']';
        } else {
            echo "--";
        }
    }
}

add_filter("manage_edit-product_sortable_columns", 'related_product_col_sort');

function related_product_col_sort($columns) {
    $custom = array(
        'RELATED' => '_related_ids'
    );
    return wp_parse_args($custom, $columns);
}