MongoDB修改數據類型，int與字符串互轉

operator_log爲集合名，operation_type爲字段名

{$type:2}代表type是2的，即字符串，查詢所有字符串的forEach，並複製，保存

db.operator_log.find({"operation_type":{$type:2}}).forEach(function(x)
{
x.operation_type=NumberInt(x.operation_type);
db.operator_log.save(x);
}
)

db.operator_log.find({"operation_type":{$exists:true}}).forEach(function(x){
    x.operation_type=x.operation_type+"";   // 用new string(x.operation_type)就變成了文檔，不知道爲什麼
    db.operator_log.save(x);
});