Re: Correct implementation of 1:n relationship with n>0?

On 04/30/2013 04:39 PM, Wolfgang Keller wrote:
> 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.

Does a solution like the following work for you? It has some plpgsql,
but you didn't do it yourself.

drop schema if exists one_n cascade;
create schema one_n;
set search_path to 'one_n';

create table lists (id serial primary key, name text);
create table list_items (
id serial primary key,
list_id integer not null
references lists (id)
on update cascade on delete cascade,
value text);
create index on list_items (list_id);

create or replace function list_cardinality_enforcer()
returns trigger as
$$
declare
__list_id integer;
begin
if tg_table_name = 'lists' then
__list_id := new.id;
elsif tg_table_name = 'list_items' then
__list_id := old.list_id;
else
raise exception 'This trigger function is only intended for
tables lists and list_items';
end if;

/* Take a lock on the list id to handle concurrency */
perform id from one_n.lists where id = __list_id for update;

if not exists (select 1 from one_n.list_items li where list_id =
__list_id) then
raise exception 'List with id=% does not have any items', __list_id;
end if;

return null;
end;
$$
language plpgsql;

create constraint trigger list_cardinality_enforcer
after insert on lists
deferrable initially deferred
for each row execute procedure list_cardinality_enforcer();

create constraint trigger list_cardinality_enforcer
after update or delete on list_items
deferrable initially deferred
for each row execute procedure list_cardinality_enforcer();

My basic tests work, it's possible I've not thought of something.