计算要从 mongodb 中嵌套引用数组的查找查询中获取的元素数

Question

我有以下分类集合

{
"_id" : ObjectId("5e1d8c3ed06661d2b06a7c46"),
    "status" : "active",
    "noOfLevels" : 3,
    "levels" : [ 
        ObjectId("5e1db2237a265cfe50dfdb18"), 
        ObjectId("5e1db22b7a265cfe50dfdb19"), 
        ObjectId("5e1db23b7a265cfe50dfdb1a")
    ],
    "name" : "Fuel",
},
{
 "_id" : ObjectId("5e1d8c3ed06661d2b06a7c46"),
    "status" : "active",
    "noOfLevels" : 3,
    "levels" : [ 
        ObjectId("5e1db2237a265cfe50dfdb18"), 
        ObjectId("5e1db22b7a265cfe50dfdb19"), 
        ObjectId("5e1db23b7a265cfe50dfdb1a")
    ],
    "name" : "Feeds",
}

在上述集合级别中，_id 已被存储为以后填充的参考。

关卡集合有这个项目：-

{
 levelName: { type: String, required: true },
  levelDesc: { type: String },
  levelNo: { type: Number, required: true },
  categoryId: { type: Mongoose.Schema.Types.ObjectId, ref: 'trainingCategories', required: true },
  videoId: [{ type: Mongoose.Schema.Types.ObjectId, ref: 'trainingVideoMaster' }],
},
{
 levelName: { type: String, required: true },
  levelDesc: { type: String },
  levelNo: { type: Number, required: true },
  categoryId: { type: Mongoose.Schema.Types.ObjectId, ref: 'trainingCategories', required: true },
  videoId: [{ type: Mongoose.Schema.Types.ObjectId, ref: 'trainingVideoMaster' }],
}

我正在使用聚合和查找来查找类别集合中的项目，在我的最终结果中，我需要特定类别的每个级别上存在的 videoIds 的计数。

需要这样的最终结果：-

 {
                        name: 'How to save fuel',
                        catNo: 1,
                        noOfLevels: 3,
                        totalVideos: 4
    },
    {
                        name: 'Feeds',
                        catNo: 2,
                        noOfLevels: 3,
                        totalVideos: 3
     }

Answer 1

您需要$unwind trainingLevels 数组使其成为对象属性，然后您可以使用.dot 表示法来获取videoId 长度。

Cat.aggregate([
  { "$match": { "status": "active" } },
  { "$lookup": {
    "from": "traininglevels",
    "let": { "levels": "$levels" },
    "pipeline": [
      { "$match": { "$expr": { "$in": ["$_id", "$$levels"] }}},
      { "$group": {
        "_id": null,
        "totalVideos": { "$sum": { "$size": "$videoId" }}
      }}
    ]
    "as": "trainingLevels"
  }},
  { "$unwind": "$trainingLevels" },
  { "$project": {
    "noOfLevels": 1,
    "name": 1,
    "catNo": 1,
    "createdAt": 1,
    "trainingLevels": "$trainingLevels.totalVideos"
  }}
])