mongoDB“；$lookup的参数必须是字符串，让：。。。。是第3类“；，

如何在id数组（长）而不是单个id的字段上执行$lookup？ 试图通过_id从集合“products”中检索产品信息，将其放入数组中，然后将产品信息列表嵌入仓库文档

不幸的是，我们的mongo数据库是3.2版本的，因此新特性允许使用array（）进行查找不适用

通过在线查看一系列解决方案，这似乎是最接近的，将其修改为以下内容：

db.getCollection('warehouses').aggregate([

   {"$match": 
        // conditions to be matched
   },
    { $lookup:
       {
         from: 'products',
         let: {'productIds' : '$productIds' },
         pipeline: [
           { $match: { $expr: {$in: ["$_id", "$$productIds"] } } },
         ],
         as: 'productLists'
       }
     }
])

但是，它会出现以下错误：

"arguments to $lookup must be strings, let: .... is type 3".

_id是数字（长）而不是字符串，是否有解决方法？谢谢

仓库文件样本：

{
    "_id" : NumberLong(1),
    productIds: [NumberLong(1), NumberLong(2), NumberLong(3)],
    "warehouseProperty1" : "warehouseProperty1",
    "warehouseProperty2" : "warehouseProperty2",
    "warehouseProperty3" : "warehouseProperty3",
    "warehouseProperty4" : "warehouseProperty4"
}

{
    "_id" : NumberLong(1),
    "productProperty1" : "productProperty1",
    "productProperty2" : "productProperty2",
    "productProperty3" : "productProperty3",
    "productProperty4" : "productProperty4"
}

产品文件样本：

{
    "_id" : NumberLong(1),
    productIds: [NumberLong(1), NumberLong(2), NumberLong(3)],
    "warehouseProperty1" : "warehouseProperty1",
    "warehouseProperty2" : "warehouseProperty2",
    "warehouseProperty3" : "warehouseProperty3",
    "warehouseProperty4" : "warehouseProperty4"
}

{
    "_id" : NumberLong(1),
    "productProperty1" : "productProperty1",
    "productProperty2" : "productProperty2",
    "productProperty3" : "productProperty3",
    "productProperty4" : "productProperty4"
}

期望输出：

{
    "_id" : NumberLong(1),
    productIds: [
        {
            "_id" : NumberLong(1),
            "productProperty1" : "productProperty1",
            "productProperty2" : "productProperty2",
            "productProperty3" : "productProperty3",
            "productProperty4" : "productProperty4"
        },
        {
            "_id" : NumberLong(2),
            "productProperty1" : "productProperty1",
            "productProperty2" : "productProperty2",
            "productProperty3" : "productProperty3",
            "productProperty4" : "productProperty4"
        },
        {
            "_id" : NumberLong(3),
            "productProperty1" : "productProperty1",
            "productProperty2" : "productProperty2",
            "productProperty3" : "productProperty3",
            "productProperty4" : "productProperty4"
        },
    ],
    "warehouseProperty1" : "warehouseProperty1",
    "warehouseProperty2" : "warehouseProperty2",
    "warehouseProperty3" : "warehouseProperty3",
    "warehouseProperty4" : "warehouseProperty4"
}

---

let

也不是3.2中存在的密钥。表示使用$unwind。

let

也不是3.2中存在的键。表示使用$REWIND。