What are the different ways of adding a constraint so that only items that are available on the order date can be inserted? - sql

Question

order.date must be between item.date_from and item.date_to... what are the different ways of doing that?
CREATE TABLE "item" (
"id" SERIAL PRIMARY KEY,
"date_from" DATE NOT NULL,
"date_to" DATE NOT NULL
);
CREATE TABLE "order" (
"id" SERIAL PRIMARY KEY,
"date" DATE NOT NULL
);
CREATE TABLE "order_item" (
"order" INTEGER NOT NULL REFERENCES "order",
"item" INTEGER NOT NULL REFERENCES "item"
);

Answer 1

Check constraints work on simple expressions. For example, a simple sanity check on the order: check( date > '2010-01-01'). There's also exclusion constraints which check no two rows have the same value as defined by the exclusion. But, with the exception of foreign key constraints, constraints don't query other tables.
You can solve this with a trigger on insert and update, and I'll go into that below, but its better to solve this sort of problem with referential integrity. However, I can't think of a way to do that.
You can make a view of available items for the order. Here $1 is the date of the order.
create temporary view items_available_to_order
select *
-- pluralize table names to avoid conflicting with keywords and columns
from items
-- date_from and date_to has become a single daterange when_available
where items.when_available #> $1
Then only insert items from that view.
If you want to go the trigger route (you can do both) write a function which checks whether an order's item is valid. It either raises an exception or returns a trigger. new is the inserted row, or the row after an update.
I changed some of the table and column names and types to avoid common pitfalls.
create function check_item_order_is_valid()
returns trigger
language 'plpgsql'
as $body$
declare
item_is_available boolean;
begin
select
items.when_available #> orders.ordered_on into item_is_available
from item_orders
join items on items.id = new.order_id
join orders on orders.id = new.item_id;
if( not item_is_available) then
raise exception 'Item #% is not available for order #%',
new.item_id, new.order_id;
end if;
return new;
end
$body$
Then define a trigger to call the function when rows are inserted or updated in the item/order table.
create trigger check_item_orders
before insert or update
on item_orders
for each row
execute function check_item_order_is_valid();
Demonstration.
What if the valid range of an item changes? You need an update trigger on item to check that its orders are still valid. Maybe. Depends on your business logic.

Answer 2

A test example:
CREATE OR REPLACE FUNCTION public.item_date()
RETURNS trigger
LANGUAGE plpgsql
AS $function$
DECLARE
order_date date;
from_date date;
to_date date;
BEGIN
select into order_date "date" from "order" where id = new.order;
select into from_date, to_date date_from, date_to from item where id = new.item;
--Use date range to test whether order date is in item date range.
if order_date <# daterange(from_date, to_date, '[]') then
return new;
else
return null;
end if;
END;
$function$
create trigger item_date_check before insert or update on order_item for each row execute function item_date();
insert into item values (1, '09/01/2021', '10/31/2021');
insert into item values (2, '07/01/2021', '08/31/2021');
insert into "order" values (1, '09/05/2021');
insert into order_item values (1, 1);
NOTICE: Order date 2021-09-05, from_date 2021-09-01, to_date 2021-10-31
INSERT 0 1
--Returning NULL causes the INSERT not to happen.
insert into order_item values (1, 2);
NOTICE: Order date 2021-09-05, from_date 2021-07-01, to_date 2021-08-31
INSERT 0 0
Note that I had to quote "order" as that is a reserved word also. You might to take a look at Key(reserved) Words. For range functions/operators see Range Function. For general information on range(s) see Range Types