It hit me today that a 1:n relationship can't be implemented just by a single foreign key constraint if n>0. I must have been sleeping very deeply not to notice this.
E.g. if there is a table "list" and another table "list_item" and the relationship can be described as "every list has at least one list_item" (and every list_item can only be part of one list, but this is trivial).
A "correct" solution would require (at least?):
1. A foreign key pointing from each list_item to its list
2. Another foreign key pointing from each list to one of its list_item. But this must be a list_item that itself points to the same list, so just a simple foreign key constraint doesn't do it.
3. When a list has more than one list_item, and you want to delete the list_item that its list points to, you have to "re-point" the foreign key constraint on the list first. Do I need to use stored proceures then for all insert, update, delete actions?
(4. Anything else that I've not seen?)
Is there a "straight" (and tested) solution for this in PostgreSQL, that someone has already implemented and that can be re-used?
No, I definitely don't want to get into programming PL/PgSQL myself. especially if the solution has to warrant data integrity under all circumstances. Such as concurrent update, insert, delete etc.
TIA,
Sincerely,
Wolfgang
I don't think there is the way to achieve that without programming (less important in which language...)
Your rules say:
1) End user - can't be able to create new list at all... (just new List) (If he can create new list - it will brake the your rule 2)
He always creates "list_item" - but in one case - should pick existing "list" in another he must enter info about new list_item together with info about new list....
Technically - create new list_item calls one or another function
2) End User - just can delete list_item (function will make additional check - if there is no more list_items in my list - delete the list as well - the same check will be run after "repoint")
Everything else - will be assured with existing FK integrity....
