Question

我有以下数据集：

{ 
    patientId: 228,
    medication: {
        atHome : [
        {
            "drug" : "tylenol", 
            "start" : "3", 
            "stop" : "7"
        }, 
        {
            "drug" : "advil", 
            "start" : "0", 
            "stop" : "2"
        }, 
        {
            "drug" : "vitaminK", 
            "start" : "0", 
            "stop" : "11"
        }
        ], 
    }
}

当我执行以下聚合时，一切看起来都很棒。

db.test01.aggregate(
[
    {$match: {patientId: 228}},
    {$project: {
        patientId: 1,
        "medication.atHome.drug": 1
        }
    },
]);

结果（正是我想要的）：

{ 
"_id" : ObjectId("5a57b7d17af6772ebf647939"), 
"patientId" : NumberInt(228), 
"medication" : {
    "atHome" : [
        {"drug" : "tylenol"}, 
        {"drug" : "advil"}, 
        {"drug" : "vitaminK"}
    ]}
}

然后我们想添加ifNull来将空值更改为默认值，但是这会破坏结果。

db.test01.aggregate(
[
    {$match: {patientId: 228}},
    {$project: {
         patientId: {$ifNull: ["$patientId", NumberInt(-1)]},
         "medication.atHome.drug": {$ifNull: ["$medication.atHome.drug", "Unknown"]}
        }
    },
]);

ifNull的结果（不是我希望的）：

{ 
"_id" : ObjectId("5a57b7d17af6772ebf647939"), 
"patientId" : NumberInt(228), 
"medication" : {
    "atHome" : [
        {"drug" : ["tylenol", "advil", "vitaminK"]}, 
        {"drug" : ["tylenol", "advil", "vitaminK"]}, 
        {"drug" : ["tylenol", "advil", "vitaminK"]}, 
    ]}
}

我缺少什么或不理解？

Answer 1

要将数组元素的文档属性设置为默认值，您需要$unwind数组，然后检查 null group >。这是查询：

db.test01.aggregate([ // unwind to evaluete the array elements {$unwind: "$medication.atHome"}, {$project: { patientId: {$ifNull: ["$patientId", -1]}, "medication.atHome.drug": {$ifNull: ["$medication.atHome.drug", "Unknown"]} } }, // group to put atHome documents to an array again {$group: { _id: {_id: "$_id", patientId: "$patientId"}, "atHome": {$push: "$medication.atHome" } } }, // project to get a document of required format {$project: { _id: "$_id._id", patientId: "$_id.patientId", "medication.atHome": "$atHome" } } ])

<强>更新还有另一个更简洁的查询来实现同样的目标。它使用map运算符来计算每个数组元素，因此不需要展开。

db.test01.aggregate([ {$project: { patientId: {$ifNull: ["$patientId", -1]}, "medication.atHome": { $map: { input: "$medication.atHome", as: "e", in: { $cond: { if: {$eq: ["$$e.drug", null]}, then: {drug: "Unknown"}, else: {drug: "$$e.drug"} } } } } } } ])

Mongodb对数组元素聚合ifNull

1 个答案: