Writing to different tables from writable view

Question

I want to write to different tables depending on the state of my development (b=backup, t=testing).

The minimal example showing the problem is the following:

SELECT s.Monat FROM (
SELECT 'b' as mode, * FROM [BACKUP].IstMonat
UNION 
SELECT 't' as mode, * FROM [TESTING].IstMonat
) s
where s.mode = 't' -- in the real world discovered by a sub query.

How can i create such a view that is actually writable?

Currently ...

INSERT INTO TestView(Monat) VALUES (1);

... results in ...

Msg 4406, Level 16, State 1, Line 1
Update or insert of view or function 'TestView' failed because it contains a derived or constant field.

Answer 1

A (non-indexed) View cannot have two potentially writable table-targets at run-time. That's one of the reasons that Views with UNION in them are automatically read-only.

I can think of only two ways that this could be done:

1. View Triggers: Create a View with an INSTEAD OF Trigger that procedureally decides which table to write to, (I cannot remember if this has to be an Indexed View or not), or ...

2. Synonyms: Define your View's target through the use of a Synonym. Then when you want to switch it, simply redefine the synonym.

Answer 2

I solved it with something like the following:

CREATE TRIGGER ModifyAllSynonyms ON [IMPORT].ImpIstMonat
INSTEAD OF INSERT
AS
IF ( EXISTS (SELECT * FROM dbo.ModeSwitch WHERE mode = 'backup') )
INSERT INTO [BACKUP].IstMonat 
SELECT * FROM inserted
ELSE
INSERT INTO [TESTING].IstMonat
SELECT * FROM inserted