How do I restrict the list of choices in one table based on user selection in another
Answer
Suppose there is a Countries table and a Cities table - the City refers to Country.
If you are implementing Addresses table, obviously it is enough to have a relation to the City - the Country could be looked up from Cities table.
However if you want the user to select Country first and then the City from the country-restricted list you should first add a relation from Addresses to a Country - this will render Countries as a dropdown - and add match condition to the Address-City relationship to match City.Country and Address.Country.
