While there are ways to generate an alert should such a thing happen, there is, to my knowledge, no way to prevent it. Currently, there is no way to limit user choice to already-existing linked records a user may create a new linked record at any selection level.

The version discussed here and in the referenced base requires only category and subcategory tables to be linked to records may be added to the main table without any such preliminary housekeeping. While variations are possible where only the final, limited selection be presented as a linked record, they impose other limitations: namely, that each record of the main table be linked to a single record in the table for conditional logic to work. This version of the routines requires all selections be made in the form of linked records.There are limitations to quote from my original announcement

While this functionality still does not exist using conditional single selects - that is, where the list of possible choices for a, and so on are presented in the form of a linked record.

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