ALTER TABLE IF EXISTS document_item
  ADD COLUMN IF NOT EXISTS version_number INTEGER;

UPDATE document_item
   SET version_number = 1
 WHERE version_number IS NULL;

ALTER TABLE IF EXISTS document_item
  ALTER COLUMN version_number SET DEFAULT 1;

ALTER TABLE IF EXISTS document_item
  ALTER COLUMN version_number SET NOT NULL;

ALTER TABLE IF EXISTS document_item
  ADD COLUMN IF NOT EXISTS version_group_id BIGINT;

UPDATE document_item
   SET version_group_id = id
 WHERE version_group_id IS NULL;

DO $$
BEGIN
  IF NOT EXISTS (
    SELECT 1
      FROM pg_constraint
     WHERE conname = 'fk_document_item_version_group'
  ) THEN
    ALTER TABLE document_item
      ADD CONSTRAINT fk_document_item_version_group
      FOREIGN KEY (version_group_id)
      REFERENCES document_item(id)
      ON DELETE SET NULL;
  END IF;
END
$$;

DO $$
BEGIN
  IF EXISTS (
    SELECT 1
      FROM pg_constraint
     WHERE conname = 'document_item_document_number_key'
       AND conrelid = 'document_item'::regclass
  ) THEN
    ALTER TABLE document_item
      DROP CONSTRAINT document_item_document_number_key;
  END IF;
END
$$;

CREATE UNIQUE INDEX IF NOT EXISTS idx_document_item_document_number_version_unique
  ON document_item (document_number, version_number);

CREATE INDEX IF NOT EXISTS idx_document_item_version_group_id
  ON document_item (version_group_id);