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How to update a multi-select with a cacluated value script

Accepted answer

"Multi select" is a property of an editor presentation that is available for attribute types [type]List where [type] is a value as mentioned above. As far as I can tell, the JavaScript Attribute Customization API does not work with attributes of type [type]List. Only Java based attribute customization or other means of extending work here. The documentation https://jazz.net/wiki/bin/view/Main/AttributeCustomization#API_for_Javascript mentions which types are supported and there is nothing mentioning any list type attributes.
Comments

Hi Ralph,
Thanks for that.
We knew the internal representation was a string, we were hoping that some formatting of some type would give us what we want. Looks like it's not going to be possible.

I can't see what formatting would have anything to do with this. Enumeration list attributes used to be represented as a string containing comma separated enumeration literal ID values (see https://rsjazz.wordpress.com/2015/02/24/extending-the-workitem-command-line-with-new-commands/ for some information on that). This was in RTC 3.x. As this is out of support, I don't consider this to be used any more. You could potentially do something with this old representation, but not for the 4.x and higher enumerationList types, as that is a list.