MongoDB報錯"not authorized on root to execute command"問題解決一例

用root使用者在修改MongoDB認證機制的時候報錯:
>use admin
>db.auth('root','123456')
> var schema=db.system.version.findOne({"_id" : "authSchema"})

Y> schema.currentVersion = 3
3
> db.system.version.save(schema)
WriteResult({
        "writeError" : {
                "code" : 13,
                "errmsg" : "not authorized on root to execute command { update: \"system.version\", updates: [ { q: { _id: \"authSchema\" }, u: { _id: \"authSchema\", currentVersion: 3 }, multi: false, upsert: true } ], ordered: true }"
        }
})

此錯誤是因為沒有授權給admin使用者對system.version表執行命令的許可權，解決方法如下:
> db.grantRolesToUser ( "root", [ { role: "__system", db: "admin" } ] )

再次執行命令成功:
> db.system.version.findOne({"_id" : "authSchema"})
{ "_id" : "authSchema", "currentVersion" : 3 }