Foreign key constraint on array member?

Question

Suppose I have a table containing job roles:

CREATE TABLE roles
(
  "role" character varying(80) NOT NULL,
  CONSTRAINT "role" PRIMARY KEY (role)
);

Suppose I further have a table, users, and each row (a specific user) can have an arbitrary number of job roles:

CREATE TABLE users
(
  username character varying(12) NOT NULL,
  roles character varying(80)[] NOT NULL,
  CONSTRAINT username PRIMARY KEY (username)
);

I should probably make sure that each member of users.roles[] exists in roles.role. It seems to me that what I want is a foreign key constraint on each member of users.roles[] such that if references roles.role.

This doesn't seem possible with postgres. Am I looking at this the wrong way? What is the suggested "right" way to handle this?

Answer 1

Support for array foreign keys was worked on with the goal of getting it into PostgreSQL 9.3, but it didn't make the cut for the release due to performance and reliability problems. It doesn't seem to be being worked on for 9.4.

At this time, you need to stick to the usual relational approach of using a "join table" to model an m:n relationship.

CREATE TABLE user_roles (
   username character varying(12) references users(username),
   "role" character varying(80) references roles("role"),
   PRIMARY KEY(username, "role")
);

I suggest using surrogate keys in this case, too, rather than storing the usernames/role names directly in the join table. The first time you want to rename a user or role you'll be happy you used surrogate keys. Just place a unique constraint on roles."role" and users.username.

Answer 2

I just made something similar for a colleague. Essentially I made a hidden table that contained one row for each (user,role) pair with suitable constraints. The user table was then a view of the hidden table with all the roles assembled into an array. I then made it possible to insert into the view by adding an appropriate rule. Here is how:

trailer=# create table harvester (id int unique, label text);
CREATE TABLE
trailer=# insert into harvester values (1,'grain'), (2,'cricket');
INSERT 0 2
trailer=# create table donkey (id int, others int references
harvester(id));
CREATE TABLE
trailer=# create unique index donkey_ears on donkey (id, others);
CREATE INDEX
trailer=# create view combine as select id, array_agg(others) as others
from donkey group by id;
CREATE VIEW
trailer=# create rule combine_insert as on insert to combine do instead
(delete from donkey where donkey.id=new.id;insert into donkey select
new.id,unnest(new.others) );
CREATE RULE
trailer=# insert into combine values (1,'{1,2}');INSERT 0 2
trailer=# select * from combine ;
id | others 
----+--------
  1 | {1,2}
(1 row)

trailer=# insert into combine values (1,'{1,2}');
INSERT 0 2
trailer=# select * from combine ;
 id | others 
----+--------
  1 | {1,2}
    (1 row)

trailer=# insert into combine values (2,'{1,2,3}');
ERROR:  insert or update on table "donkey" violates foreign key
constraint "donkey_others_fkey"
DETAIL:  Key (others)=(3) is not present in table "harvester".
trailer=# 

I hope that helps. You can make it a bit more efficient and add more rules depending on your requirements.

Answer 3

Once you get the patch that allows that functionality more here

Just use: ELEMENT REFERENCES relation( field )

For intance:

CREATE TABLE drivers (
   driver_id integer PRIMARY KEY,
   first_name text,
   last_name text,
   ...
);



CREATE TABLE races (
   race_id integer PRIMARY KEY,
   title text,
   race_day DATE,
   ...
   practice1_positions integer[] ELEMENT REFERENCES drivers,
   practice2_positions integer[] ELEMENT REFERENCES drivers,
   practice3_positions integer[] ELEMENT REFERENCES drivers,
   qualifying_positions integer[] ELEMENT REFERENCES drivers,
   final_positions integer[] ELEMENT REFERENCES drivers
);