migrate63 error

migrate63.md

This query

INSERT INTO "event_user_notification_sent" (
    "id", "ts", "notification"
) (
    SELECT "id", "ts", "notification"
	FROM "event_notification_sent"
	WHERE "id" IN (
        SELECT "id"
		FROM "user_notification"
	)
);

fails with

ERROR:  insert or update on table "event_user_notification_sent" violates foreign key constraint "event_user_notification_sent_notification_fkey"
DETAIL:  Key (notification)=(4) is not present in table "user_notification".

Let's look at the constraint in question:

snowdrift_development=# \d event_user_notification_sent;
                                     Table "public.event_user_notification_sent"    Column    |           Type           |                                 Modifiers                                 
--------------+--------------------------+---------------------------------------------------------------------------
 id           | integer                  | not null default nextval('event_user_notification_sent_id_seq'::regclass)
 ts           | timestamp with time zone | not null
 notification | bigint                   | not null
Indexes:
    "event_user_notification_sent_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "event_user_notification_sent_notification_fkey" FOREIGN KEY (notification) REFERENCES user_notification(id)

The last line means that "each 'notification' field must correspond to the 'id' field of the 'user_notification' table.

But why does it need an 'id = 4'?

SELECT "id" FROM "user_notification";

-[ RECORD 1 ]
id | 2
-[ RECORD 2 ]
id | 3