MS SQL “ON DELETE CASCADE” multiple foreign keys pointing to the same table?

Question

Howdy, I have a problem where i need a cascade on multiple foreign keys pointing to the same table..

[Insights]
| ID | Title        |
| 1  | Monty Python |
| 2  | Spamalot     | 

[BroaderInsights_Insights]
| broaderinsight_id | insight_id |
| 1                 | 2          |

Basically when either record one or two in the insights table is deleted i need the relationship to also be deleted..

I've tried this:

 CREATE TABLE broader_insights_insights(id INT NOT NULL IDENTITY(1,1),
   broader_insight_id INT NOT NULL REFERENCES insights(id) ON DELETE CASCADE,
   insight_id INT NOT NULL REFERENCES insights(id) ON DELETE CASCADE,
   PRIMARY KEY(id))
Go

This results in the warning that the cascade "may cause cycles or multiple cascade path"

So ive tried adding a cascade to just the insight_id and this results in:

"The DELETE statement conflicted with the REFERENCE constraint"

Any ideas?

Thanks

Daniel

Answer 1

You'll have to implement this as an INSTEAD OF delete trigger on insights, to get it to work. Something like:

create trigger T_Insights_D
on Insights
instead of delete
as
    set nocount on
    delete from broader_insights_insights
    where insight_id in (select ID from deleted) or
    broader_insight_id in (select ID from deleted)

    delete from Insights where ID in (select ID from deleted)

---

Frequently with cascading deletes and lots of foreign keys, you need to spend time to work out a "cascade" order so that the delete that occurs at the top of a "tree" is successfully cascaded to referencing tables. But that isn't possible in this case.