PostgreSQL check constraint for foreign key condition - sql

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.

Answer 2

The only way I can think of is to add a unique constraint on (id, superuser) to the users table and reference that from the user_has_job table by "duplicating" the superuser flag there:
create table users (
id serial primary key,
name text not null,
superuser boolean not null default false
);
-- as id is already unique there is no harm adding this additional
-- unique constraint (from a business perspective)
alter table users add constraint uc_users unique (id, superuser);
create table job (
id serial primary key,
description text
);
create table user_has_job (
user_id integer references users (id),
-- we need a column in order to be able to reference the unique constraint in users
-- the check constraint ensures we only reference superuser
superuser boolean not null default true check (superuser),
job_id integer references job(id),
constraint user_has_job_pk PRIMARY KEY (user_id, job_id),
foreign key (user_id, superuser) references users (id, superuser)
);
insert into users
(id, name, superuser)
values
(1, 'arthur', false),
(2, 'ford', true);
insert into job
(id, description)
values
(1, 'foo'),
(2, 'bar');
Due to the default value, you don't have to specify the superuser column when inserting into the user_has_job table. So the following insert works:
insert into user_has_job
(user_id, job_id)
values
(2, 1);
But trying to insert arthur into the table fails:
insert into user_has_job
(user_id, job_id)
values
(1, 1);
This also prevents turning ford into a non-superuser. The following update:
update users
set superuser = false
where id = 2;
fails with the error
ERROR: update or delete on table "users" violates foreign key constraint "user_has_job_user_id_fkey1" on table "user_has_job"
Detail: Key (id, superuser)=(2, t) is still referenced from table "user_has_job".

Answer 3

Create a separate superuser table that inherits from the user table:
CREATE TABLE "user" (
id serial PRIMARY KEY,
name text NOT NULL,
);
CREATE TABLE superuser () INHERITS ("user");
The user_has_job table can then reference the superuser table:
CREATE TABLE user_has_job (
user_id integer REFERENCES superuser (id),
job_id integer REFERENCES job(id),
PRIMARY KEY (user_id, job_id)
);
Move users around between the tables as needed by inserting and deleting:
WITH promoted_user AS (
DELETE FROM "user" WHERE id = 1 RETURNING *
) INSERT INTO superuser (id, name) SELECT id, name FROM promoted_user;

Answer 4

I don't know if this is a good way to do it but it seems to work
INSERT INTO user_has_job (user_id, job_id) VALUES (you_user_id, your_job_id)
WHERE EXIST (
SELECT * FROM user WHERE id=your_user_id AND superuser=true
);