def get_last_tid(self):
        try:
            spec = {'tid': {'$gt': 0}}
            sort = [('tid', -1)]
            row = self.db.docs.find_one(spec, sort=sort)
        except Exception as e:
            raise DatabaseException(e)
        if row:
            return row['tid']
        return 0
def dfs(self,can,cursum,target,res):
        if cursum > target: return 
        if cursum == target: 
            self.result.append(res)
            return 
        for i in xrange(len(can)):
            if not res or res[len(res)-1] <= can[i]:
                self.dfs(can,cursum+can[i],target,res+[can[i]])