def insert(self, doc, synced=False):
        doc.id = self.get_next_id()
        doc.synced = synced
        try:
            self.db.docs.insert(doc)
        except Exception as e:
            raise DatabaseException(e)
        return doc
def combinationSum(self, candidates, target):
        self.result = []
        self.dfs(candidates,0,target,[])
        return self.result