我需要用猫鼬填充来自同一集合的数据

Question

我有一个名为Exam的收藏集。 我需要将数据填充到以下代码所示的同一集合中。 下面是我的路线的代码。

    router.get('/fetchanswerbyid/:id',
    auth,
    async (req,res)=>{
        try {
            let user = await Exam.find({"_id":req.params.id}).populate('answers.user.answer.question_id', Exam) //Exam is the name of Exam model
            res.json(user)
        } catch (error) {
            console.error(error.message)

        }
    })

下面是我的模型的代码。

const mongoose = require('mongoose')

const ExamSchema = new mongoose.Schema({

    classroom:{
        type:mongoose.Schema.Types.ObjectId,
        ref:'classrooms'
    },
    exam_name:{
        type: String,
        required: true
    },
     subject:{
        type: String,
        required: true
    },
    questions:[
        {
            question:{
            type: String,
            required: true
            }, 
        answerA:{
                   type: String,

                   },
        answerB:{
                   type: String,

                  },

        answerC:{
                   type: String,

                   },

        answerD:{
                   type: String,

                 },
         checkedA:{
                  type: Boolean,
                  default: false 

        },
           checkedB:{
                  type: Boolean,
                  default: false 

        },
           checkedC:{
                  type: Boolean,
                  default: false 

        },
           checkedD:{
                  type: Boolean,
                  default: false 

        },
        pattern:{
             type: String,
             required: false
        },
        queType:{
             type: String,
             required: false

        },
     textAns:{
               type: String
             },
        }],

        answers:[{

            user:[{
                user_id:{
                    type:mongoose.Schema.Types.ObjectId,
                    ref:'user_collection'
                },
                answer:[{
                question_id:{
                    type:mongoose.Schema.Types.ObjectId,
                    refPath: "exam"
                },
                checkedA:{
                    type: Boolean,
                    default: false 

                    },
                checkedB:{
                        type: Boolean,
                        default: false 

                    },
                checkedC:{
                        type: Boolean,
                        default: false 

                    },
                checkedD:{
                        type: Boolean,
                        default: false 

                    },
                textAns:{
                    type: String
                    },
            }]
            }]
        }]


    })
module.exports = mongoose.model('exam',ExamSchema)

当我请求带有考试ID的路线时，我希望从上面的“问题”数组中填充questions_id（在“答案”数组中）的数据。 目前，这就是我的输出在邮递员中的样子。

[
    {
        ...   
        "subject": "AI / ML",
        "exam_name": "a",
        ...
        "questions": [
            {
                "checkedA": true,
                "checkedB": false,
                "checkedC": false,
                "checkedD": false,
                "_id": "5ee13e82169454eec35b7408",
                "question": "abc",
                "marks": "12",
                .....

            }
        ],
        "__v": 0,
        "answers": [
            {
                "user": [
                    {
                        "answer": [
                            {
                                "checkedA": false,
                                "checkedB": false,
                                "checkedC": false,
                                "checkedD": false,
                                "_id": "5ee5419d0d63bfc75e81b45b",
                                "question_id": null
                            }
                        ],
                        "_id": "5ee5419d0d63bfc75e81b45a",
                        "user_id": "5ed22a3a6e8451061f4e1855"
                    }
                ],
                "_id": "5ee5419d0d63bfcs214d2259"
            }
        ]
    }
]

对于“ question_id”，其显示为“ null”，应在同一集合中填充问题数组。

有人可以帮我吗？