DO $$
BEGIN
  IF NOT EXISTS (
    SELECT 1
    FROM pg_constraint
    WHERE conname = 'bills_source_invoice_id_fkey'
      AND conrelid = 'public.bills'::regclass
  ) THEN
    ALTER TABLE public.bills
    ADD CONSTRAINT bills_source_invoice_id_fkey
    FOREIGN KEY (source_invoice_id)
    REFERENCES public.invoices(id)
    ON DELETE SET NULL;
  END IF;
END $$;