How use cascade delete for array field in PostgreSQL?

Question

How can i remove the value from array field? I need to delete the user id from the university table's user_id array field if exists when i delete any users. so can i do this by using delete cascade or any other method? please help me on this

Table:

CREATE TABLE IF NOT EXISTS users
    (_id SERIAL NOT NULL UNIQUE PRIMARY KEY,
    "userName" VARCHAR NOT NULL,
     active BOOLEAN NOT NULL);

CREATE TABLE IF NOT EXISTS university
    (_id SERIAL NOT NULL UNIQUE PRIMARY KEY,
    "universityName" VARCHAR NOT NULL,
    user_id integer[]);

User:

| _id | userName | active |
===========================
| 1   |  Abc     |   true |
| 2   |  vty     |   true |
| 7   |  hyu     |   true |
| 9   |  Agc     |   true |

University:

| _id | universityName    | user_id |
=====================================
| 1   |  HGV              |  {1,9}  |
| 2   |  CPC              |   {2}   |

1) if i delete user 2 then entire row should get deleted from university 2) if i delete user 1 then only value 1 should be removed from the user_id field from university table

Answer 1

First of all you need to create gin index on array field:

CREATE INDEX index_university_user_id ON university USING gin (user_id);

Then you need to add trigger to process deleting users:

CREATE FUNCTION trg_d_users() RETURNS TRIGGER AS $f$
BEGIN
    UPDATE university
       SET user_id = array_remove(user_id, OLD.id)
     WHERE user_id = OLD.id;
    RETURN OLD; 
END
$f$ LANGUAGE plpgsql;

CREATE TRIGGER trg_d_users BEFORE DELETE ON users FOR EACH ROW EXECUTE PROCEDURE trg_d_users()

If you want to remove university without users, you can add into function after UPDATE:

DELETE FROM university WHERE user_id = '{}';

And also I would reccomend to rename user_id field to user_ids. It very useful to see that field is array without checking schema.