code: def transaction(self):
          
          # The idea here is to fake the nesting of transactions. Only when
          # we've gotten back to the topmost transaction context do we actually
          # commit or rollback.
          with self.mdr:
              try:
                  self._depth += 1
                  yield self
                  self._depth -= 1
              except self.mdr.OperationalError:
                  # We've lost the connection, so there's no sense in
                  # attempting to roll back back the transaction.
                  self._depth -= 1
                  raise
              except:
                  self._depth -= 1
                  if self._depth == 0:
                      self.mdr.rollback()
                  raise
              if self._depth == 0:
                  self.mdr.commit()
docs: Sets up a context where all the statements within it are ran within
          a single database transaction. For internal use only.