1,错误做法，直接更新表

> db.system.users.update({"_id" : ObjectId("529e67553992b24438d5e315")},{"user":"tank2","readOnly" : false,"pwd":"123"}) 
> db.system.users.find(); 
{ "_id" : ObjectId("529e5f8474b4c660718a70f3"), "user" : "tank1", "readOnly" : false, "pwd" : "35dd47abff098f5b4f0b567db8edeac5" } 
{ "_id" : ObjectId("529e67553992b24438d5e315"), "user" : "tank2", "readOnly" : false, "pwd" : "123" } //这样就不对了