sql - PostgreSQL check constraint for foreign key condition

Question

I have a table of users eg:

create table "user" (
    id serial primary key,
    name text not null,
    superuser boolean not null default false
);

and a table with jobs:

create table job (
    id serial primary key,
    description text
);

the jobs can be assigned to users, but only for superusers. other users cannot have jobs assigned.

So I have a table whereby I see which job was assigned to which user:

create table user_has_job (
    user_id integer references "user"(id),
    job_id integer references job(id),
    constraint user_has_job_pk PRIMARY KEY (user_id, job_id)
);

But I want to create a check constraint that the user_id references a user that has user.superuser = True.

Is that possible? Or is there another solution?

Answer 1

This would work for INSERTS:

create or replace function is_superuser(int) returns boolean as $$
select exists (
    select 1
    from "user"
    where id   = $1
      and superuser = true
);
$$ language sql;

And then a check contraint on the user_has_job table:

create table user_has_job (
    user_id integer references "user"(id),
    job_id integer references job(id),
    constraint user_has_job_pk PRIMARY KEY (user_id, job_id),
    constraint chk_is_superuser check (is_superuser(user_id))
);

Works for inserts:

postgres=# insert into "user" (name,superuser) values ('name1',false);
INSERT 0 1
postgres=# insert into "user" (name,superuser) values ('name2',true);
INSERT 0 1

postgres=# insert into job (description) values ('test');
INSERT 0 1
postgres=# insert into user_has_job (user_id,job_id) values (1,1);
ERROR:  new row for relation "user_has_job" violates check constraint "chk_is_superuser"
DETAIL:  Failing row contains (1, 1).
postgres=# insert into user_has_job (user_id,job_id) values (2,1);
INSERT 0 1

However this is possible:

postgres=# update "user" set superuser=false;
UPDATE 2

So if you allow updating users you need to create an update trigger on the users table to prevent that if the user has jobs.