如何通过避免 mongodb 聚合中的空值来进行 $lookup

Question

在这里，我使用 $lookup 到其他集合的左连接，查询工作正常，但是当某些记录缺少值时，它会返回

errmsg : $in requires an array as a second argument, found: null

查询文档结构如下：

{
 "no" : "2020921008981",
 "sale" : {
  "soldItems" : [
    {
        "itemId" : "5b55ac7f0550de00210a3b24", 
    },

    {
        "itemId" : "5b55ac7f0550de00215584re", 
    }
  ], 
 "bills" : [
    {
        "billNo" : "2020921053467", 
        "insurancePlanId" : "160", 
    },

    {
        "billNo" : "2020921053467", 
        "insurancePlanId" : "170", 
     }
   ],
   "visitIds" : [
   5b55ac7f0550de00210a3b24, 5b55ac7f0550de00210a3b24
   ]

  }
}

查询：

db.case.aggregate([
{
    $lookup: {
        from: "insurance",
        let: { ipids: "$sale.bill.insurancePlanId" },
        pipeline: [
            {
                $unwind: "$coveragePlans"
            },
            {
                $match: { $expr: { $in: ["$coveragePlans._id", "$$ipids"] } }
            },
            {
                $project: { _id: 0, name: 1 }
            }
        ],
        as: "insurances"
    }
},
{
    $lookup: {
        from: "item",
        let: { iid: "$salesOrder.purchaseItems.itemRefId" },
        pipeline: [
            {
                $match: {
                    $expr: {
                        $in: ["$_id", {
                            $map: {
                                input: "$$iid",
                                in: { $toObjectId: "$$this" }
                            }
                        }
                        ]
                    }
                }
            }
        ],
        as: "items"
      }
  }
])

保险收取：

{ 
  "_id" : ObjectId("5b55aca20550de00210a6d25"), 
  "name" : "HIJKL" 
  "coveragePlans" : [
    {
      "_id" : "160", 
      "name" : "UVWZ", 
    }, 
    { 
    "_id" : "161", 
    "name" : "LMNO", 
    }
   ]
 },
{ 
  "_id" : ObjectId("5b55aca20550de00210a6d25"),  
  "name" : "WXYZ"
  "coveragePlans" : [
   {
    "_id" : "169", 
    "name" : "5ABC", 
   }, 
   { 
    "_id" : "170", 
    "name" : "4XYZ", 
    }
  ]
}

物品收藏：

{ 
  "_id" : ObjectId("5b55ac7f0550de00210a3b24"), 
  "code" : "ABCDE"
},
{ 
  "_id" : ObjectId("5b55ac7f0550de00215584re"), 
  "code" : "PQRST" 
}

如何避免这种情况并在管道进入下一阶段之前有效地进行空检查？尝试过，{ $match: { "fieldName": { $exists: true, $ne: null } } }但它返回有关格式的 mongo 错误。如果这是要走的路，请提及我应该放置的阶段..提前致谢

Answer 1

您可以使用$ifNull运算符

let: { ipids: {$ifNull:["$sale.bill.insurancePlanId", [] ]} },

编辑：跳过空"$salesOrder.purchaseItems.itemRefId"值

let: { iid: {$filter: {input:"$salesOrder.purchaseItems.itemRefId", cond:{$ne:["$$this", ""]}}} },