Reselect item on Select2 multi-value select list - oracle-apex

Question

I use plugin Select2 on Apex.
I have scenario like this:
I have three tables: ROOM, MASTER_STUDENT and MAP_STUDENT_ROOM
One ROOM can have many STUDENT
User can select more than one student(with Select2 from MASTER_STUDENT) when create ROOM
When user edit ROOM, previously selected student show in Select2 item(selected item by MAP_STUDENT_ROOM), so user can remove or add more Student
How to achieve point number 4, item Select2 list of values is MASTER_STUDENT but with default selected by MAP_STUDENT_ROOM?
I found this documentation, but i dont know how to apply it.

Answer 1

I'm not familiar with the Select2 plug-in.
Have you considered using the built-in Popup LOV, which since APEX 19.1 now allows multiple selection.

Answer 2

Whether you're using the Select2 plug-in or the built-in Popup LOV, the issue will be the same. You have a form on ROOM, that's trying to take into account an item that isn't part of the ROOM table (its values are stored in MAP_STUDENT_ROOM). The trick is to populate the item correctly during page load so that the item can correctly display the currently assigned students.
How is the item's Source configured? If it's not, set Type to SQL Query (return colon separated value). Assuming you have a primary key item on the page for the room (e.g. P1_ID), enter a query like the following:
select student_id
from MAP_STUDENT_ROOM
where room_id = :P1_ID
Then set Used to Always, replacing any existing value in session state.
This should get the item to display correctly when the page loads. However, you'll still have to figure out how to map the values back to the MAP_STUDENT_ROOM table correctly when the page is submitted. You'll need to add some logic that first deletes rows from the room (P1_ID) that are not in the selection (e.g. P1_ASSIGNED_STUDENTS). You can use APEX_STRING.SPLIT_NUMBERS to help:
delete from map_student_room
where room_id = :P1_ID
and student_id not in (
select column_value
from apex_string.split_number(:P1_ASSIGNED_STUDENTS, ':')
)
Then you can insert rows that are in the selection but not already in the room.
insert into map_student_room (
room_id,
student_id
)
select :P1_ID,
column_value
from apex_string.split_number(:P1_ASSIGNED_STUDENTS, ':')
where column_value not in (
select student_id
from map_student_room
where room_id = :P1_ID
)
I've not tested any of this code, but it should get the points across.
Another option would be to use an Interactive Grid instead of a list of values item. This is a classic master/detail scenario where students could be added and removed below the form region (typically only done after create).