mongodb aggregate多表联查多阶数组嵌套查询实现

let aggregate=[
  {
    $lookup:
      /**
       * from: The target collection.
       * localField: The local join field.
       * foreignField: The target join field.
       * as: The name for the results.
       * pipeline: Optional pipeline to run on the foreign collection.
       * let: Optional variables to use in the pipeline field stages.
       */
      {
        from: "class",
        localField: "_id",
        foreignField: "schoolId",
        as: "class",
      },
  },
  {
    $unwind:
      /**
       * path: Path to the array field.
       * includeArrayIndex: Optional name for index.
       * preserveNullAndEmptyArrays: Optional
       *   toggle to unwind null and empty values.
       */
      {
        path: "$class",
        includeArrayIndex: "classIndex",
        preserveNullAndEmptyArrays: false,
      },
  },
  {
    $addFields:
      /**
       * newField: The new field name.
       * expression: The new field expression.
       */
      {
        classId: {
          $toString: "$class._id",
        },
      },
  },
  {
    $lookup:
      /**
       * from: The target collection.
       * localField: The local join field.
       * foreignField: The target join field.
       * as: The name for the results.
       * pipeline: Optional pipeline to run on the foreign collection.
       * let: Optional variables to use in the pipeline field stages.
       */
      {
        from: "student",
        localField: "classId",
        foreignField: "classId",
        as: "student",
      },
  },
  {
    $addFields:
      /**
       * newField: The new field name.
       * expression: The new field expression.
       */
      {
        "class.students": "$student",
      },
  },
  {
    $group: {
      _id: "$_id",
      classes: {
        $push: "$class",
      },
    },
  },
  {
    $lookup:
      /**
       * from: The target collection.
       * localField: The local join field.
       * foreignField: The target join field.
       * as: The name for the results.
       * pipeline: Optional pipeline to run on the foreign collection.
       * let: Optional variables to use in the pipeline field stages.
       */
      {
        from: "school",
        localField: "_id",
        foreignField: "_id",
        as: "school",
      },
  },
  {
    $unwind:
      /**
       * path: Path to the array field.
       * includeArrayIndex: Optional name for index.
       * preserveNullAndEmptyArrays: Optional
       *   toggle to unwind null and empty values.
       */
      {
        path: "$school",
      },
  },
  {
    $project:
      /**
       * specifications: The fields to
       *   include or exclude.
       */
      {
        _id: 0,
        id: {
          $toString: "$_id",
        },
        classes: 1,
        name: "$school.name",
      },
  },
]
this.schoolModel.aggregate(aggregate);

let aggregate=[
    {
     $lookup:
      {
        from: "class",
        localField: "_id",
        foreignField: "schoolId",
        as: "classes",
        pipeline:[
         {
          $lookup:{
           from: "student",
           localField: "classId",
           foreignField: "classId",
           as: "students",
           } 
         }
        ]
      },   
  ]
this.schoolModel.aggregate(aggregate);