How to convert sql_variant so it can be considered deterministic - sql-server

Question

I am trying to create a persisted computed column in a SYSTEM_VERSIONING table dbo.Users as follows:
ALTER TABLE dbo.Users
ADD SessionId AS usr.GetSession() PERSISTED
CONSTRAINT FK_dboUsers_IdSession
FOREIGN KEY REFERENCES dbo.Sessions(IdSession)
Where usr.GetSession() is just retrieving the value stored as BIGINT in SESSION_CONTEXT('IdSession') and converting it to again to BIGINT.
CREATE OR ALTER FUNCTION usr.GetSession()
RETURNS BIGINT WITH SCHEMABINDING
AS
BEGIN
RETURN CONVERT(BIGINT, SESSION_CONTEXT(N'IdSession'))
END
But getting the following error:
Computed column 'SessionId' in table 'Users' cannot be persisted because the column is non-deterministic.
It is obviously because:
SELECT OBJECTPROPERTY(OBJECT_ID('usr.GetSession'), 'IsDeterministic') AS IsDeterministic;
Is returning 0
A little bit of searching found this about Deterministic and nondeterministic functions
CONVERT
Deterministic unless one of these conditions exists:
Source type is sql_variant.
Target type is sql_variant and its source type is nondeterministic.
So, I am understanding that there is no way to make my computed column persisted with a user defined scalar function as sql_variant cannot be handled as deterministic value.
Or there can be some walk around to solve my problem? Or some other solution? Any idea?

Answer 1

No, there is no workaround. You cannot do anything with sql_variant unless you convert it (even implicitly), and as you mention, that is not deterministic.
Be that as it may, it seems you are going down the wrong road anyway.
A computed column is the wrong thing here, as in this case it would change every time it was read, whereas it seems you want it changed every time the row is inserted.
Instead you need a DEFAULT
ALTER TABLE dbo.Users
ADD SessionId bigint DEFAULT (usr.GetSession())