-- A transaction imported twice cannot simply be deleted.
--
-- Every child of `transactions` is ON DELETE CASCADE — splits, tags, overrides,
-- expense_metadata, order_reviews. Deleting a row said to be "the duplicate"
-- therefore destroys whatever curation happens to sit on it, silently and
-- unrecoverably. The curation is not reliably on the surviving side either: of
-- the 31 known duplicate pairs, one carries splits and six carry overrides, and
-- which member holds them is an accident of import order.
--
-- So a duplicate is superseded, never removed. The row stays, keeps its
-- children, and points at the row that replaces it. Reversing a mistake is then
-- one UPDATE rather than a restore from backup.
--
-- This is the statement-vs-statement case. `reconciled_with_id` already covers
-- manual-vs-statement, and deliberately cannot be reused: the predicate that
-- hides a reconciled row is scoped to `statement_id IS NULL`, because a
-- statement line pointing at something else is the survivor, not the duplicate.
-- Both of these rows are statement lines.

ALTER TABLE transactions
  ADD COLUMN IF NOT EXISTS superseded_by_id integer
    REFERENCES transactions(id) ON DELETE SET NULL;

COMMENT ON COLUMN transactions.superseded_by_id IS
  'This row was imported twice; the named row is the one that counts. Excluded from every figure, kept for its children and its audit trail. NULL = live.';

CREATE INDEX IF NOT EXISTS idx_transactions_superseded
    ON transactions (superseded_by_id)
 WHERE superseded_by_id IS NOT NULL;

-- A row cannot supersede itself, and a survivor cannot itself be superseded
-- (that would hide both members of the pair and lose the amount entirely).
ALTER TABLE transactions
  DROP CONSTRAINT IF EXISTS transactions_no_self_supersede;
ALTER TABLE transactions
  ADD CONSTRAINT transactions_no_self_supersede
  CHECK (superseded_by_id IS NULL OR superseded_by_id <> id);