Help to write a field handler similar to CCK's content_handler_field_multiple

With the excellent views advanced help and this issue, I've managed to create a working view using hook_views_data(). However I still need to create a custom handler something like views_handler_field_one2many. Perhaps this issue could become a mini howto.

A simplified version of what I have is: Each row in tour_booking has one or more matching rows in tour_booking_group.

mysql> desc tour_booking;
+---------------------------+----------------------+------+-----+---------+----------------+
| Field                     | Type                 | Null | Key | Default | Extra          |
+---------------------------+----------------------+------+-----+---------+----------------+
| bkid                      | int(10) unsigned     | NO   | PRI | NULL    | auto_increment |
| date_created              | int(10) unsigned     | NO   | MUL | 0       |                |
 
mysql> desc tour_booking_group;
+-----------+------------------+------+-----+---------+-------+
| Field     | Type             | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| bkid      | int(10) unsigned | NO   | PRI | 0       |       |
| delta     | int(10) unsigned | NO   | PRI | 0       |       |
| name1     | varchar(255)     | NO   |     | NULL    |       |

Here is the code of the hook:

function tour_booking_views_data() {
	$data = array();

	$data['tour_booking'] = array(
		'table'	=> array(
			'group'		=> t("Tour booking"),
			'base'		=> array(
				'field'		=> 'bkid',
				'title'		=> t("Tour booking"),
				'help'		=> t("Tour's base booking table"),
				//'weight'	=> 0,
			),
		),

		'bkid' => array (
			'title'		=> t("BKID"),
			'help'		=> t("Booking id: primary key"),
			'field' => array(
				'handler'			=> 'views_handler_field_numeric',
				'click sortable'	=> TRUE,
			),
			'filter' => array(
				'handler' => 'views_handler_filter_numeric',
			),
			'sort' => array(
				'handler' => 'views_handler_sort',
			),
		),

		'date_created' => array (
			'title'		=> t("Date created"),
			'help'		=> t("When the booking was placed"),
			'field' => array(
				'handler' => 'views_handler_field_date',
				'click sortable' => TRUE,
			),
			'sort' => array(
				'handler' => 'views_handler_sort_date',
			),
			'filter' => array(
				'handler' => 'views_handler_filter_date',
			),
		),

		//plus many more field definitions
	);

	$data['tour_booking_group'] = array(
		'table'	=> array(
			'group'		=> t("Tour booking"),
			'join'		=> array(
				'tour_booking' => array(
					'left_field'	=> 'bkid',
					'field'			=> 'bkid',
				),
			),
		),

		'bkid' => array (
			'title'		=> t("BKID"),
			'help'		=> t("1st part of groups primary key"),
		),

		'delta' => array (
			'title'		=> t("Group delta"),
			'help'		=> t("2nd part of groups primary key"),
			'field' => array(
				'handler'			=> 'views_handler_field_numeric', // *** here ***
			),
			'sort' => array(
				'handler' => 'views_handler_sort',
			),
		),

		'name1' => array (
			'title'		=> t("Group name 1"),
			'help'		=> t("Group member's first name"),
			'field' => array(
				'handler' => 'views_handler_field', // *** and here ***
			),
		),

		//plus many more field definitions
	);

	return $data;
}

At present I get a complete row of all fields for each of the rows in the second table tour_booking_group.
What I would like is all of the rows from tour_booking_group that match a row in tour_booking, to be part of that row.
E.G.:
BKID: 1
Date created: 19.01.2010
Delta: 0, 1, 2, 3
Group name 1: John, Mary, Hans, Eva
BKID: 2
etc.

This is possible with CCK when using fields with multiple values. My guess is the CCK custom handler for this is content_handler_field_multiple. It's beyond my knowledge to fathom how it works.

My guess is I will need to create a class that extends views_handler_field using handler->pre_query() to pre-fetch the sub-rows from tour_booking_group.

I would very much appreciate any help and pointers anybody could offer.

Thanks Richard