猫鼬聚合如何对同一集合中2个数组中的2个对象求和

Question

我正在尝试在一系列信贷和债务中求和，但结果却是错误的（信用：800，借方：8）是400和4。我不知道我在做什么错。

我的收藏

{
    "_id": "5ca2156960e3fa38a0087cd8",
    "name": "Jan",
    "data": "2019-01-01T02:00:00.000Z",
    "__v": 18,
    "credits": [
        {
            "_id": "5ca24eabbc5e01341c034ecf",
            "name": "cred 1",
            "valor": 200
        },
        {
            "_id": "5ca24ea9bc5e01341c034ece",
            "name": "cred 2",
            "valor": 200
        },
    ],
    "debits": [
        {
            "_id": "5ca230cffc553e36ec5841dd",
            "name": "debit1",
            "value": 2,
            "status": "OK"
        },
        {
            "_id": "5ca230cafc553e36ec5841dc",
            "name": "debit2",
            "value": 2,
            "status": "OK"
        }] }

代码：

db.collection.aggregate([
{ $unwind: '$credits' },
{ $unwind: '$debits' },
{$project: {
credits: { $sum: '$credits.value' },
debits: { $sum: '$debits.value' }}},
{
$group: {
_id: null,
credits: { $sum: '$credit' },
debits: { $sum: '$debits' }
}},
{$project: {
id: 0,
credits: 1,
debits: 1
}}])