Агрегировать и раскручивать массив, но держать ключ верхнего уровня

Question

Допустим, у меня есть следующий документ в моей коллекции Classes Collecton

  {
    "_id": ObjectId("5df58d45244a850d54b922c8"),
    "mentors" : {
        "numOfMentors" : NumberInt(1), 
        "mentorList" : [
            ObjectId("5c9ba636347bb645e0865283")
        ]
    },
    "lessons": [
      {
        "_id": ObjectId("5db221be211d7b68ac8be618"),
        "mentorData": {
          "objective": "Ensuring students that making mistakes is normal and that it is a part of life",
          "task": "Post a video explaining obstacles that you had to overcome as a programmer",
          "dueDate": "2019-12-14T15:26:10.000+0000"
        },
        "studentData": {
          "objective": "Learning that failures help you grow",
          "task": "Post a video explaining obstacles that you have overcame",
          "dueDate": "2020-01-14T22:26:10.000+0000" <---- CHECKING THIS DATE
        },
        "title": "How to overcome obstacles",
        "comments": []
      }
    ]    
  }

И я агрегирую следующим образом:

  Class.aggregate([
            { $match: { "students.studentList": req.user._id } },
            { $unwind: "$lessons" },
            {
              $addFields: {
                date: {
                  $dateToString: {
                    format: "%Y-%m-%d",
                    date: "$lessons.studentData.dueDate"
                  }
                }
              }
            },
            {
              $match: {
                $and: [
                  { date: { $gte: startDate } },
                  { date: { $lte: endDate } }
                ]
              }
            },
            { $group: { _id: "$_id", lessons: { $push: "$lessons" } } }
          ])

, который возвращает это ...

 {
            "lessonImage": {
                "url": "https://stemsandbox.blob.core.windows.net/stemuli/lesson-picture-a406bd19-0677-4b60-909e-7de2ca3c6f93.jpg",
                "originalname": "nintendo-switch-console.jpg",
                "mimetype": "image/jpeg",
                "blobName": "lesson-picture-a406bd19-0677-4b60-909e-7de2ca3c6f93.jpg",
                "container": "stemuli",
                "blob": "lesson-picture-a406bd19-0677-4b60-909e-7de2ca3c6f93.jpg",
                "size": "147469",
                "etag": "\"0x8D797B8F1EC9C39\"",
                "createdOn": "2020-01-12T23:41:28.588Z"
            },
            "_id": "5db221be211d7b68ac8be619",
            "mentorData": {
                "objective": "Learn to make a single web page web app",
                "task": "Create a short video instructing how to setup environment",
                "dueDate": "2019-02-02T22:26:10.000Z"
            },
            "studentData": {
                "objective": "Program a single page web app in React",
                "task": "Program a single page web app in React and submit by october 30th",
                "dueDate": "2020-01-22T22:26:10.000Z"
            },
            "title": "Learning to program in React",

        }

Я хочу, чтобы это было так Примечание mentors поле

 {
            "lessonImage": {
                "url": "https://stemsandbox.blob.core.windows.net/stemuli/lesson-picture-a406bd19-0677-4b60-909e-7de2ca3c6f93.jpg",
                "originalname": "nintendo-switch-console.jpg",
                "mimetype": "image/jpeg",
                "blobName": "lesson-picture-a406bd19-0677-4b60-909e-7de2ca3c6f93.jpg",
                "container": "stemuli",
                "blob": "lesson-picture-a406bd19-0677-4b60-909e-7de2ca3c6f93.jpg",
                "size": "147469",
                "etag": "\"0x8D797B8F1EC9C39\"",
                "createdOn": "2020-01-12T23:41:28.588Z"
            },
            "_id": "5db221be211d7b68ac8be619",
            "mentorData": {
                "objective": "Learn to make a single web page web app",
                "task": "Create a short video instructing how to setup environment",
                "dueDate": "2019-02-02T22:26:10.000Z"
            },
            "studentData": {
                "objective": "Program a single page web app in React",
                "task": "Program a single page web app in React and submit by october 30th",
                "dueDate": "2020-01-22T22:26:10.000Z"
            },
            "title": "Learning to program in React",
            "classId": "5e1baea87fcee8639cbce29d",\
          //FIELD BELOW ADDED
            "mentors": [ 
                ObjectId("5c9ba636347bb645e0865283")
                  ]
        }

Answer 1

Вы можете добавить $push к вашему $group, а затем запустить $ уменьшение в качестве следующего этапа для выравнивания массива массивов:

{ $group: { _id: "$_id", lessons: { $push: "$lessons" }, mentors: { $push: "$mentors.mentorList" } } },
{ $addFields: { mentors: { $reduce: { input: "$mentors", initialValue: [], in: { $setUnion: [ "$$value", "$$this" ] } } } } }

Пн go Детская площадка

РЕДАКТИРОВАТЬ: $ setUnion удалит все возможные дубликаты