ALTER TABLE match_participants
    ADD COLUMN disconnected_at TIMESTAMPTZ;

-- The pre-lease connection receipt populated connected_at but did not advance
-- the already-present generation column. Preserve those live admissions as
-- generation one before enforcing the lease invariant.
UPDATE match_participants
SET connection_generation = 1
WHERE connected_at IS NOT NULL AND connection_generation = 0;

ALTER TABLE match_participants
    ADD CONSTRAINT match_participants_connection_lease
    CHECK (
        (connection_generation = 0 AND connected_at IS NULL AND disconnected_at IS NULL)
        OR
        (connection_generation > 0 AND connected_at IS NOT NULL)
    ) NOT VALID;