Validation rule looking at prior value on related object

Question

Related to my previous question: How to make accounts read-only / locked?

I'm trying to prevent changes from being made to accounts with a certain record type.

One possible change a user could make is to the list of related objects (e.g. its contacts). Accordingly, I'm trying to write a validation rule for these object types that prevents them from being either added to or removed from these records. I thought the following would work:

AND(
    OR(
        Account.RecordTypeId = "0124E0000000fKD",
        PRIORVALUE(Account.RecordTypeId) = "0124E0000000fKD"
    ),
    OR(
        ISCHANGED(AccountId),
        ISNEW()
    )
)

But a get a syntax error:

The PRIORVALUE function cannot reference the Account.RecordTypeId field

Some Googling reveals that others have had similar problems (e.g. this thread).

So: what's the right way to check the record type of the previously linked account?

Answer 1

The Account isn't being modified, so checking its previous value doesn't make any sense. It didn't change! Just remove that part of your formula:

AND(
    Account.RecordTypeId = "0124E0000000fKD",
    OR(
        ISCHANGED(AccountId),
        ISNEW()
    )
)

I recommend you also filter on RecordType.DeveloperName. It makes your validation less brittle as it should work in all your environments, and also much easier to read:

Account.RecordType.DeveloperName = "Some_Record_Type_You_Can_Read"