-- Emoji reactions on chat messages.
--
-- One reaction per person per message: reacting again REPLACES rather than
-- accumulates, which is what the primary key enforces. That matches how the
-- UI reads — a row of distinct emoji with counts — and stops someone stacking
-- twenty reactions on one bubble.
--
-- The emoji is stored as text rather than a code point or shortname: what the
-- sender picked is what everyone should see, and a mapping table would need
-- updating every time the emoji set grows.
CREATE TABLE IF NOT EXISTS message_reactions (
    message_id      UUID        NOT NULL REFERENCES messages(id) ON DELETE CASCADE,
    user_id         UUID        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    -- Denormalized so deleting a conversation takes its reactions with it
    -- without relying on the messages cascade alone.
    conversation_id UUID        NOT NULL REFERENCES conversations(id) ON DELETE CASCADE,
    emoji           VARCHAR(32) NOT NULL,
    created_at      TIMESTAMPTZ NOT NULL DEFAULT now(),
    updated_at      TIMESTAMPTZ NOT NULL DEFAULT now(),
    PRIMARY KEY (message_id, user_id),
    -- A stray empty string would render as an invisible reaction chip.
    CONSTRAINT message_reactions_emoji_not_blank CHECK (btrim(emoji) <> '')
);

-- Hydrating a page of messages: one index range scan per batch.
CREATE INDEX IF NOT EXISTS message_reactions_message_idx
    ON message_reactions (message_id);