“无法识别的表达式'$ addFields'”

Question

我收到错误：

  

“无法识别的表达式'$ addFields'”

当我尝试使用MongoDB聚合框架计算数组中字段的平均值时。

有人可以帮我理解下面的代码段中的错误。

我的数据库版本是3.4.5

"doctor": {

        "$map": {
            "input": "$$procedure.doctor",
            "as": "doctor",
            "in": {
                "$cond": [//specify the filter for doctor
                    {
                        "$eq": ["$$doctor.activeFlag", "Y"]
                    },
                    {
                        "doctorName": "$$doctor.doctorName", 

                        "DoctorUserRating": {

                            "$map": {
                                "input": "$$doctor.DoctorUserRating",
                                "as": "docUserRate",
                                "in": {
                                    "$cond": [//specify the filter for user rating
                                    {},
                                    {
                                     //Calculate the average of userRating from array
                                        "$addFields": {
                                            "avarageDocRating": {
                                                "$divide": [
                                                    { // expression returns total
                                                        "$reduce": {
                                                            "input": "$$docUserRate",
                                                            "initialValue": 0,
                                                            "in": { "$add": ["$$value", "$$this.userRating"] }
                                                        }
                                                    },
                                                    { // expression returns ratings count
                                                        "$cond": [
                                                            { "$ne": [{ "$size": "$$docUserRate" }, 0] },
                                                            { "$size": "$$docUserRate" },
                                                            1
                                                        ]
                                                    }
                                                ]
                                            }
                                        },

                                    //////////////////////
                                        },
                                        false
                                    ],
                                },//"in": {
                            },//"$map": {
                        },//"DoctorUserRating": {          
                    },
                    false
                ],
            },//"in": {
        },//"$map": {
    },//"doctor": 

我的模特是

hospitalName: { type: String, required: true, trim: true },
hospitalID: { type: Number, required: true, unique: true, dropDups: true }, 
serviceActiveFlag: { type: String, required: true, enum: ['Y', 'N'] },//new    
Treatment: [{        
    procedureid: { type: Number },

    costUpperBound: { type: Number, required: true },                

    doctor: [{
        doctorId: { type: Number},
        doctorName: { type: String, required: true, trim: true },                    
        DoctorUserRating:[ {  
            userRating:{type: Number, required: true, //To get default rating for cost api
                min: [1, 'The value of path `{PATH}` ({VALUE}) is beneath the limit ({MIN}).'],
                max: [5, 'The value of path `{PATH}` ({VALUE}) exceeds the limit ({MAX}).']                                       
            },
            userId: { type: String, required: false, trim: true},
        }],
   }],
}],
updated_at: { type: D

吃，必需：true，默认：Date.now}

Answer 1

我找到了解决方案 -

"DoctorUserRating": {

"$map": {
    "input": "$$doctor.DoctorUserRating",
    "as": "docUserRate",
    "in": {
        "$cond": [//specify the filter for user rating
        {},
        {
         "averageDocRate": { "$avg": "$$docUserRate.userRating" }
        },
 }