'Get the latest block for a given public key
:param public_key: The public_key for which the latest block has to be found.
:return: the latest block or None if it is not known'

def get_latest(self, public_key):

    return self._get((u'WHERE    public_key    =    ?    AND    sequence_number    =    (SELECT    MAX(sequence_number)    FROM    %s    WHERE    public_key    =    ?)' % self.db_name), (buffer(public_key), buffer(public_key)))
