ALTER TABLE attachments DROP CONSTRAINT attachments_message_id_fkey;

ALTER TABLE messages ALTER COLUMN id DROP DEFAULT;
ALTER TABLE messages ALTER COLUMN id TYPE uuid USING gen_random_uuid();
ALTER TABLE messages ALTER COLUMN id SET DEFAULT gen_random_uuid();

ALTER TABLE attachments ALTER COLUMN message_id TYPE uuid USING gen_random_uuid();
ALTER TABLE attachments ADD CONSTRAINT attachments_message_id_fkey
    FOREIGN KEY (message_id) REFERENCES messages(id) ON DELETE CASCADE;