Есть такая бд:

employee:
+----+----------------+-------------------+-------------+
| id | name           | email             | skype_id    |
+----+----------------+-------------------+-------------+
|  1 | Ivan Petrov    | petrov@mail.ru    | ivan_petrov |
|  2 | Dmitry Voronov | dmitry@mail.ru    | dm_dm       |
|  3 | Elena Smirnova | smirnova@area9.dk | smir-el     |
+----+----------------+-------------------+-------------+

project:
+----+---------+-------------+
| id | name    | description |
+----+---------+-------------+
|  1 | Drive   | d           |
|  2 | Essence | e           |
|  3 | AdWords | a           |
+----+---------+-------------+


allocation:
+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+
|  * | *    |        * |       * | *                |
+----+------+----------+---------+------------------+

Вот так сейчас работает обычный дропдаун:

=== CASE 1 ===

// меняем выбранный элемент дропдауна в репорте
+----------------+--------+			+----------------+---------+
|                | week 1 |			|                | week 1  |
+----------------+--------+	===>	+----------------+---------+
| Elena Smirnova | Pick   |			| Elena Smirnova | AdWords |
+----------------+--------+			+----------------+---------+

// изменения в бд
Add record:
+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+
|  1 | 1    |        3 |       3 | NULL             |
+----+------+----------+---------+------------------+

=== CASE 2 ===

+-------------+--------+			+-------------+---------+
|             | week 1 |			|             | week 1  |
+-------------+--------+	===>	+-------------+---------+
| Ivan Petrov | Drive  |			| Ivan Petrov | AdWords |
+-------------+--------+			+-------------+---------+

Update record (work_description остается прежним):
+----+------+----------+---------+------------------+			+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |			| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+	===>	+----+------+----------+---------+------------------+
|  1 | 1    |        1 |       1 | d1               |			|  1 | 1    |        1 |       3 | d1               |
+----+------+----------+---------+------------------+			+----+------+----------+---------+------------------+

Так будет если добавить мультиселект дропдаун:

=== CASE 1 ===

+----------------+--------+			+----------------+----------------+
|                | week 1 |			|                | week 1         |
+----------------+--------+	===>	+----------------+----------------+
| Elena Smirnova | Pick   |			| Elena Smirnova | Drive, AdWords |
+----------------+--------+			+----------------+----------------+

Add record:
+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+
|  1 | 1    |        3 |       1 | NULL             |
|  2 | 1    |        3 |       3 | NULL             |
+----+------+----------+---------+------------------+


=== CASE 2 ===

+-------------+--------+			+-------------+---------+
|             | week 1 |			|             | week 1  |
+-------------+--------+	===>	+-------------+---------+
| Ivan Petrov | Drive  |			| Ivan Petrov | AdWords |
+-------------+--------+			+-------------+---------+

Delete record:
+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+
|  1 | 1    |        1 |       1 | d1               |
+----+------+----------+---------+------------------+

Add record:
+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+
|  2 | 1    |        1 |       3 | NULL             |
+----+------+----------+---------+------------------+

=== CASE 3 ===

allocation:
+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+
|  1 | 1    |        1 |       1 | d1               |
|  2 | 1    |        1 |       2 | d2               |
+----+------+----------+---------+------------------+

+-------------+-----------------+			+-------------+------------------+
|             | week 1          |			|             | week 1           |
+-------------+-----------------+	===>	+-------------+------------------+
| Ivan Petrov | Drive, Essence  |			| Ivan Petrov | Essence, AdWords |
+-------------+-----------------+			+-------------+------------------+

Delete record:
+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+
|  1 | 1    |        1 |       1 | d1               |
+----+------+----------+---------+------------------+

Add record:
+----+------+----------+---------+------------------+
| id | week | employee | project | work_description |
+----+------+----------+---------+------------------+
|  3 | 1    |        1 |       3 | NULL             |
+----+------+----------+---------+------------------+


Проблема в том, что в case2 и case3 теряется work_description. Использовать в case2 UPDATE вместо DELETE+ADD — не вариант, поскольку вместо одного AdWords может быть выбранно несколько проектов (как в case3). И тут уже ставить для всех записей work_description = d1 неправильно, т.к. в удаленных записях может быть и d1, и d2.