db.cr

  @@transactions = Hash(PG::Connection, Int32).new { 0 }

  def self.transaction
    transaction_id = Random::DEFAULT.next_u32

    App.db do |db|
      transaction_counter = @@transactions[db]

      if transaction_counter == 0
        db.exec("BEGIN")
      else
        db.exec("SAVEPOINT txn#{transaction_id}")
      end
      @@transactions[db] = transaction_counter + 1

      begin
        yield db

        @@transactions[db] = transaction_counter
        if transaction_counter == 0
          db.exec("COMMIT")
        else
          db.exec("RELEASE SAVEPOINT txn#{transaction_id}")
        end
      rescue e
        @@transactions[db] = transaction_counter
        if transaction_counter == 0
          db.exec("ROLLBACK")
        else
          db.exec("ROLLBACK TO SAVEPOINT txn#{transaction_id}")
        end

        raise e
      end
    end
  end