'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'

def __init__(self, cls, db_alias):

    self.cls = cls
    self.collection = cls._get_collection()
    self.db_alias = db_alias
    self.ori_db_alias = cls._meta.get('db_alias', DEFAULT_CONNECTION_NAME)
