Update boolean in a related table

[SaschaValentin]

My concern may be a little difficult to understand, but I'll try to describe it.

I have three tables: Two with ten fields each (Field 01...Field 10) and another with the Name fields and two relationships to the other two tables. Users should enter their name using a form and then be able to select the fields from the other two tables using a select field. Only the values ​​that are still free should be displayed in the select field - i.e. book free slots, so to speak.

Unfortunately you cannot filter for related fields in the select field. That's why I created another field as Boolean. If the slot/field is occupied, the value should be checked. But how do I achieve this automatically when a new entry is created via the form? Wouldn't this have to work via automation and update row? But there is always an error message about invalid inputs.