postgres constraint based on SELECT condition - postgresql

Question

ᕼello! I think I have a somewhat tricky postgres situation:
parents have children. children have an age, and a flag that they are the appreciated.
The rule: a parent can't appreciate two children of the same age!
My question is: how to enforce this rule?
Current schema:
CREATE TABLE parent (
id SERIAL PRIMARY KEY,
name VARCHAR(50) NOT NULL
);
CREATE TABLE child (
id SERIAL PRIMARY KEY,
parent INTEGER REFERENCES parent(id) NOT NULL,
name VARCHAR(50) NOT NULL,
age INTEGER NOT NULL,
appreciated BOOLEAN NOT NULL
);
Put some values in:
INSERT INTO parent(name) VALUES
('bob'), -- assume bob's id = 0
('mary'); -- assume mary's id = 1
INSERT INTO child(parent, name, age, appreciated) VALUES
(0, 'child1', 10, FALSE), -- Bob has children 1, 2, 3
(0, 'child2', 10, FALSE),
(0, 'child3', 15, FALSE),
(1, 'child4', 20, FALSE), -- Mary has children 4, 5, 6
(1, 'child5', 20, FALSE),
(1, 'child6', 10, FALSE);
All fine so far. No child is appreciated, which is always valid.
Mary is allowed to appreciate child6:
UPDATE child SET appreciated=TRUE WHERE name='child6';
Bob is allowed to appreciate child2. child2 is the same age as child6 (who is already appreciated), but child6 is not Bob's child.
UPDATE child SET appreciated=TRUE WHERE name='child2';
Bob now cannot appreciate child1. This child1 is the same age as child2, and child2 is already appreciated.
UPDATE child SET appreciated=TRUE WHERE name='child2'; -- This needs to FAIL!
How do I enforce such a constraint? I'm open to all kinds of solutions, but modifying the general schema is not an option.
Thanks in advance!

Answer 1

How about a UNIQUE partial index, like so:
CREATE UNIQUE INDEX ON child(parent,age) WHERE appreciated;
So every pair of parent,age has to be unique, but only when appreciated children are considered.

Answer 2

You might want to use a trigger that activates BEFORE the insert/update and that fails if the constraint you create is not satisfied.
I suppose it should be like
create trigger <trigger_name>
before insert or update on <table_name>
for each row
declare
dummy number;
begin
select count(*)
into dummy
from <table_name>
where (appreciated=TRUE and :new.child = child and :new.age = age);
if dummy > 0 then
raise_application_error(-20001,'Too many appreciated children');
end if;
end;
Some documentation

Answer 3

The simplest thing I would think to do is add a flag grateful(?) == false to the parent model and when child.appreciated == true { parent.grateful == true }
Check the value of parent.grateful in the function that acts on child.appreciated.
If parent.grateful == true
return "sorry this parent has already shown their appreciation."
LOL this is an interesting concept though. Good Luck. :)