Looks for the most recent paste by a given nick.
        Returns the uid or None

def lookup(self, nick):
        """Looks for the most recent paste by a given nick.
        Returns the uid or None"""
        query = dict(nick=nick)
        order = [('time', pymongo.DESCENDING)]
        recs = self.db.pastes.find(query).sort(order).limit(1)
        try:
            return next(recs)['uid']
        except StopIteration:
            pass