python通过mongoengine中connect函数连接多个数据库

mongoengine支持程序同时连接多个数据库，这些数据库可以位于一个或多个mongo之中，通过alias名称区分不同的连接即可。

可以通过switch_db切换到不同的数据库，进行读写操作，switch_db其实是一个上下文管理器，通过和with语句一起使用，能够保证切换数据库的影响范围。

由于个人电脑上没有安装mongodb，以下代码示例中访问的mongodb通过mongoengine提供的mock mongodb替代，只需要在connect函数参数中加上is_mock=True参数，并且安装有mongomock package(pip install mongomock)即可。

 #!/usr/bin/env python
 # coding=utf-8
 from mongoengine import connect
 from mongoengine.document import Document
 from mongoengine.fields import StringField, IntField
 from mongoengine.context_managers import switch_db

 # 连接不同db
 connect(alias='testdbA', is_mock=True)
 connect(alias='testdbB', is_mock=True)
 connect(alias='testdbC', is_mock=True)

 class User(Document):
     meta = {
             # db_alias用于指定User绑定的mongo连接，和connect函数中的alias对应
             'db_alias': 'testdbA',
             }
     uname = StringField(max_length=63)
     uid = StringField(max_length=64)
     age = IntField(default=0)

 def create_record(uname, uid, age):
     user = User()
     user.uname = uname
     user.uid = uid
     user.age = age
     return user

 def print_records(records):
     # 输出User类此时绑定的db连接名称，并输出其记录个数
     records = list(User.objects.all())
     print('{} count:{}, records:'.format(User._meta['db_alias'], len(records)))
     for rc in records:
         print('    {}|{}|{}'.format(rc['uname'], rc['uid'], rc['age']))

 records = list(User.objects.all())
 print_records(records)
 create_record('Ace', 'AX001', 10).save()
 print_records(records)

 # switch到testdbB连接
 with switch_db(User, 'testdbB'):
     print_records(records)
     create_record('Bob', 'BX001', 11).save()
     print_records(records)

 # switch到testdbC连接
 with switch_db(User, 'testdbC'):
     print_records(records)
     create_record('Carl', 'CX001', 12).save()
     print_records(records)

运行结果如下：

$ python mongo_test.py
testdbA count:0, records:
testdbA count:1, records:
    Ace|AX001|10
testdbB count:0, records:
testdbB count:1, records:
    Bob|BX001|11
testdbC count:0, records:
testdbC count:1, records:
    Carl|CX001|12

switch_db其实就是一个简单的上下文管理器，源码位于mongoengine/context_managers.py当中，其实就是在switch_db对象初始化时、进入with代码块前、离开with代码块后，通过改变db_alias和collection两个字段对应的取值，实现了不同mongodb连接的切换，代码简单注释版本如下:

 class switch_db(object):
     """switch_db alias context manager.

     Example ::

         # Register connections
         register_connection('default', 'mongoenginetest')
         register_connection('testdb-1', 'mongoenginetest2')

         class Group(Document):
             name = StringField()

         Group(name='test').save()  # Saves in the default db

         with switch_db(Group, 'testdb-1') as Group:
             Group(name='hello testdb!').save()  # Saves in testdb-1
     """

     def __init__(self, cls, db_alias):
         """Construct the switch_db context manager

         :param cls: the class to change the registered db
         :param db_alias: the name of the specific database to use
         """
         self.cls = cls
         # 存储切换前的collection
         self.collection = cls._get_collection()
         # 设置为要切换的db连接名称
         self.db_alias = db_alias
         # 存储切换前的原db连接名称
         self.ori_db_alias = cls._meta.get('db_alias', DEFAULT_CONNECTION_NAME)
     def __enter__(self):
         """Change the db_alias and clear the cached collection."""
         # 进入with代码块前触发
         # 修改为本次指定的db连接名称
         self.cls._meta['db_alias'] = self.db_alias
         # 将collection置为None，于是之后会重新根据新的db连接获取collection
         self.cls._collection = None
         return self.cls

     def __exit__(self, t, value, traceback):
         """Reset the db_alias and collection."""
         # 退出with代码块后触发
         # 恢复为切换前的db连接名称
         self.cls._meta['db_alias'] = self.ori_db_alias
         # 恢复为切换前的collection
         self.cls._collection = self.collection