How to create a Conditional foreign key in PostgreSQL? - sql

Question

I have the following table
create table if not exists pgroup (
id uuid primary key default gen_random_uuid(),
label varchar not null,
is_role boolean default false
);
I want to create a table like the following :
create table if not exists grouprole (
groupId uuid not null references pgroup(id) `where is_role = false`,
roleId uuid not null references pgroup(id) `where is_role = true`,
primary key (groupId, roleId)
);
the idea is that two pgroup can be in a grouprole relationship if one is a role and the other is not.
My goal is that at INSERT time a check will be performed to ensure that.
EDIT:
I cannot split pgroup in two different tables because other tables references it and don't care for the is_role flag.

Answer 1

Try to use an auxiliary function in CHECK constraint:
create table if not exists pgroup (
id int primary key,
label varchar not null,
is_role boolean default false
);
create table if not exists grouprole (
groupId int not null references pgroup(id),
roleId int not null references pgroup(id),
primary key (groupId, roleId)
);
CREATE FUNCTION check_pgroup(p_id int,p_is_role boolean) RETURNS int AS $$
SELECT id
FROM pgroup
WHERE id=p_id
AND is_role=p_is_role
$$ LANGUAGE SQL;
alter table grouprole add check(check_pgroup(groupId,false) is not null);
alter table grouprole add check(check_pgroup(roleId,true) is not null);
Test:
INSERT INTO pgroup(id,label,is_role)VALUES(1,'1',true);
INSERT INTO pgroup(id,label,is_role)VALUES(2,'2',false);
INSERT INTO grouprole(groupId,roleId)VALUES(1,2); -- Error
INSERT INTO grouprole(groupId,roleId)VALUES(2,1); -- OK
You also can create a cross-check into pggroup to prevent to set a bad value into is_role:
CREATE FUNCTION check_pgroup_is_role(p_id int,p_is_role boolean) RETURNS boolean AS $$
SELECT true is_exists
FROM grouprole
WHERE ((p_is_role=true AND groupId=p_id) OR (p_is_role=false AND roleId=p_id))
$$ LANGUAGE SQL;
ALTER TABLE pgroup ADD CHECK(check_pgroup_is_role(id,is_role) IS NULL);
Test:
UPDATE pgroup SET is_role=false; -- Error
INSERT INTO pgroup(id,label,is_role)VALUES(3,'3',true); -- OK
UPDATE pgroup SET is_role=false WHERE id=3; -- OK