old_contents: # -*- coding: utf-8 -*-
  
  from . import SessionStorage
  from werobot.utils import json_loads, json_dumps
  
  
  class MongoDBStorage(SessionStorage):
      """
      MongoDBStorage 会把你的 Session 数据储存在一个 MongoDB Collection 中 ::
  
          import pymongo
          import werobot
          from werobot.session.mongodbstorage import MongoDBStorage
  
          collection = pymongo.MongoClient()["wechat"]["session"]
          session_storage = mongodbstorage.MongoDBStorage(collection)
          robot = werobot.WeRoBot(token="token", enable_session=True,
                                  session_storage=session_storage)
  
  
      你需要安装 ``pymongo`` 才能使用 MongoDBStorage 。
  
      :param collection: 一个 MongoDB Collection。
      """
      def __init__(self, collection):
          import pymongo
          assert isinstance(collection,
                            pymongo.collection.Collection)
          self.collection = collection
          collection.create_index("wechat_id")
  
      def _get_document(self, id):
          return self.collection.find_one({"wechat_id": id})
  
      def get(self, id):
          document = self._get_document(id)
          if document:
              session_json = document["session"]
              return json_loads(session_json)
          return {}
  
      def set(self, id, value):
          document = self._get_document(id)
          session = json_dumps(value)
          if document:
              document["session"] = session
              self.collection.save(document)
          else:
              self.collection.insert({
                  "wechat_id": id,
                  "session": session
              })
  
      def delete(self, id):
          document = self._get_document(id)
          if document:
              self.collection.remove(document["_id"])
  
new_contents: # -*- coding: utf-8 -*-
  
  from . import SessionStorage
  from werobot.utils import json_loads, json_dumps
  
  
  class MongoDBStorage(SessionStorage):
      """
      MongoDBStorage 会把你的 Session 数据储存在一个 MongoDB Collection 中 ::
  
          import pymongo
          import werobot
          from werobot.session.mongodbstorage import MongoDBStorage
  
          collection = pymongo.MongoClient()["wechat"]["session"]
          session_storage = MongoDBStorage(collection)
          robot = werobot.WeRoBot(token="token", enable_session=True,
                                  session_storage=session_storage)
  
  
      你需要安装 ``pymongo`` 才能使用 MongoDBStorage 。
  
      :param collection: 一个 MongoDB Collection。
      """
      def __init__(self, collection):
          import pymongo
          assert isinstance(collection,
                            pymongo.collection.Collection)
          self.collection = collection
          collection.create_index("wechat_id")
  
      def _get_document(self, id):
          return self.collection.find_one({"wechat_id": id})
  
      def get(self, id):
          document = self._get_document(id)
          if document:
              session_json = document["session"]
              return json_loads(session_json)
          return {}
  
      def set(self, id, value):
          document = self._get_document(id)
          session = json_dumps(value)
          if document:
              document["session"] = session
              self.collection.save(document)
          else:
              self.collection.insert({
                  "wechat_id": id,
                  "session": session
              })
  
      def delete(self, id):
          document = self._get_document(id)
          if document:
              self.collection.remove(document["_id"])
  
subject: correct codes in MongoDBStorage's docstring