-- Subscriptions: a logged-in user follows someone else's shared list or a
-- single shared item, to receive the same price-drop emails the owner gets.
-- Exactly one of list_id / item_id is set per row. A list subscription
-- implicitly covers every item on that list — present and future — expanded
-- at notify time rather than materialised per item.
CREATE TABLE subscriptions (
    id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    user_id    UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    list_id    UUID REFERENCES lists(id) ON DELETE CASCADE,
    item_id    UUID REFERENCES items(id) ON DELETE CASCADE,
    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
    CONSTRAINT subscription_target_exactly_one
        CHECK ((list_id IS NOT NULL) <> (item_id IS NOT NULL))
);

-- One subscription per (user, list) and per (user, item).
CREATE UNIQUE INDEX subscriptions_user_list
    ON subscriptions (user_id, list_id) WHERE list_id IS NOT NULL;
CREATE UNIQUE INDEX subscriptions_user_item
    ON subscriptions (user_id, item_id) WHERE item_id IS NOT NULL;

-- Per-subscriber, per-item de-dupe latch for target-price alerts — the
-- subscriber-side mirror of items.notified_at (which latches the owner).
-- Present = already announced this drop; absent = armed. Cleared when the
-- price climbs back above the item's target.
CREATE TABLE subscription_notify (
    user_id     UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    item_id     UUID NOT NULL REFERENCES items(id) ON DELETE CASCADE,
    notified_at TIMESTAMPTZ NOT NULL DEFAULT now(),
    PRIMARY KEY (user_id, item_id)
);