Suppose you had about 20 Choices in your multi select field and they came from a table of choices.
I suggest that you create an automation or a pipeline which triggers when the record is added or modified and that multi select field changes.
The first step will be to delete all of the existing join child records for that record which happen to exist already.
Then create a join record for every one of the 20 choices.
Then you would have a look up field which would look up the multi select field from the parent down to the drain table children. You would create a checkbox field there which would flag whether that existing join record is valid given the selection above in the multi select field on the parent.
So then the last step of the automation or the pipeline would be to delete any of the 20 join records which were created which are not valid.
I suppose it's a bit brute force to be creating all these records and then probably deleting most of them but it's kind of the stupidest simplest foolproof way to do it in my mind. You may just have to make your users aware that it will take a little bit of time for this cleanup to happen. If you are using a pipeline by the way there is a feature called bulk record delete which you can use to do the deletions.
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Mark Shnier (YQC)
mark.shnier@gmail.com
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