我正在研究MongoDB（4.0.0版）聚合功能，但是我无法通过简单的$ toString正常工作。当我删除$ toString运算符时，我得到以下输出：
{ "_id" : ObjectId("5b4fa13c0ec5e844757333d5"), "payment_date" : 20180621, "payment_date_formatted" : 20180721 }
{ "_id" : ObjectId("5b4fa13c0ec5e844757333d7"), "payment_date" : 20180626, "payment_date_formatted" : 20180726 }
但是当我尝试转换字段payment_date_formatted时，一切都出错了，输出是：
{ "_id" : ObjectId("5b4fa13c0ec5e844757333d5"), "payment_date" : 20180621, "payment_date_formatted" : "2.01807e+07" }
{ "_id" : ObjectId("5b4fa13c0ec5e844757333d7"), "payment_date" : 20180626, "payment_date_formatted" : "2.01807e+07" }
为什么会这样？这是汇总调用：
db.payment_orders.aggregate([
{
"$match": {
"company": ObjectId("5b368de558b52c13789edddd"),
"payment_date": {
"$gte": 20170601
}
}
},
{
"$addFields": {
"payment_date_formatted": {
"$add": [
"$payment_date",
100
]
}
}
},
{
"$project": {
"payment_date": 1,
"payment_date_formatted": {$toString: "$payment_date_formatted"}
}
}
]);