-- Customer favorites: one row per (user, product) or (user, shop).
-- Two nullable FKs plus a check keep the target shape honest; partial unique
-- indexes enforce uniqueness without letting NULL shop/product repeat.

CREATE TABLE favorites (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    user_id UUID NOT NULL REFERENCES users (id) ON DELETE CASCADE,
    product_id UUID REFERENCES products (id) ON DELETE CASCADE,
    shop_id UUID REFERENCES shops (id) ON DELETE CASCADE,
    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
    CONSTRAINT favorites_exactly_one_target CHECK (
        (product_id IS NOT NULL AND shop_id IS NULL)
        OR (product_id IS NULL AND shop_id IS NOT NULL)
    )
);

CREATE INDEX favorites_user_created_idx ON favorites (user_id, created_at DESC);

CREATE UNIQUE INDEX favorites_user_product_idx
    ON favorites (user_id, product_id)
    WHERE product_id IS NOT NULL;

CREATE UNIQUE INDEX favorites_user_shop_idx
    ON favorites (user_id, shop_id)
    WHERE shop_id IS NOT NULL;