<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def create_ticket_str(self, prefix=None):
 """ Generate a sufficiently opaque ticket string to ensure the ticket is not guessable. If a prefix is provided, prepend it to the string. """

        if not prefix:
            prefix = self.model.TICKET_PREFIX
        return "%s-%d-%s" % (prefix, int(time.time()),
                             get_random_string(length=self.model.TICKET_RAND_LEN))