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I found that at least SQLServer supports indexes with criteria like this: | I found that at least SQLServer supports indexes with criteria like this: | ||
CREATE unique NONCLUSTERED INDEX IX_test ON ConsolidationRequest | |||
(WinnerID) where WinnerID is not null | |||
Dailybuilds onwards does this – but since not all DB’s like it, there is a flag in the DBConfig; SupportsFilteredIndex. | Dailybuilds onwards does this – but since not all DB’s like it, there is a flag in the DBConfig; SupportsFilteredIndex. | ||
BoringPredictableServer sets this to true for MSSql, but not for SQLCe of MySql. | BoringPredictableServer sets this to true for MSSql, but not for SQLCe of MySql. | ||
[[Category:OCL]] | [[Category:OCL]] | ||
{{Edited|July|12|2024}} | |||
Latest revision as of 15:49, 10 February 2024
When you have a 1-to-1 relation between classes, a small (e) appears on the embedded link (where the key is stored).
Technically, you can store the key on both ends, but that is sort of asking for trouble - nothing good comes of it. The recommendation is to have only one (e) in the association.
A single link is implemented in the same way as a multilink – i.e a foreign key on one end. The fact that it is a single link is handled further up in the model logic.
This has left a bug I have been unable to track down yet – a single link pointing to two or more objects.
When I found this fact in a running system, I started to think about how to stop that from happening. The obvious way is to add a unique constraint to the foreign key when it is a 1-1 link, but a straight-up unique constraint will disallow multiple nulls which is not what we want or need – since null is on if the association is 0..1-1.
I found that at least SQLServer supports indexes with criteria like this:
CREATE unique NONCLUSTERED INDEX IX_test ON ConsolidationRequest
(WinnerID) where WinnerID is not null
Dailybuilds onwards does this – but since not all DB’s like it, there is a flag in the DBConfig; SupportsFilteredIndex.
BoringPredictableServer sets this to true for MSSql, but not for SQLCe of MySql.