$ lookup不返回空数组mongodb

Question

我正在创建具有聚合的mongodb，它应该在其中加入两个表，所以我使用了$ lookup，但查询返回了我一个空数组 这是我的查询

return userScoringModel.aggregate([
        {
            "$match": {
                examId: new ObjectId(req.params.examId),
                user_Id: new ObjectId(req.params.userId),
                subject: subject,
                correctMark: {$lt: 0}
            }
        },
        {
            $lookup: {
                from: "examPortalQuestions",
                localField: "questionId",
                foreignField: "_id",
                as: "question_text"
            }
        },
        {
            "$group": {
                "_id": {chapter: '$chapter'},
                "questionNumbers": {$push: "$questionNo"},
                "question_text": {$push: '$question_text'}
            }
        },
        {
            "$project": {
                "_id": 0,
                "subject": {
                    "name": "$_id.subject",
                    "chapter": "$_id.chapter",
                    "questionNumber": "$questionNumbers",
                    "question": "$question_text"
                }
            }
        }
    ]).exec()

这是输出

{
    "subject": {
        "chapter": "NUMBER TEST AND PAIR FINDING",
        "questionNumber": [
            "20"
        ],
        "question": [
            []
        ]
    }
},

样品采集 得分表：

_id,      examid,   userid ,   questionId,                    chapter
objectId  ObjectId  ObjectId   ObjectId (ref:'questiontable') XYZ
objectId  ObjectId  ObjectId   ObjectId (ref:'questiontable') qwe

问题表：

_id,       question,
ObjectId   what is abc?

我在这里问题数组为空 有人可以建议我如何将其存档 预先感谢