code: def commit(self):
          
          self._check_state()
          database = self._session._database
          api = database.spanner_api
          metadata = _metadata_with_prefix(database.name)
          txn_options = TransactionOptions(read_write=TransactionOptions.ReadWrite())
          response = api.commit(
              self._session.name,
              self._mutations,
              single_use_transaction=txn_options,
              metadata=metadata,
          )
          self.committed = _pb_timestamp_to_datetime(response.commit_timestamp)
          return self.committed
docs: Commit mutations to the database.
  
          :rtype: datetime
          :returns: timestamp of the committed changes.