我有以下具有两个字段的架构：

-finalGrade由用户输入

-numericGrade源自finalGrade

问题是：无论我从finalGrade枚举中输入哪个值，numericalGrade始终为164。当我尝试转义this.finalGrade的值时，我发现它始终返回“未定义”。

如何获取正确的值？

谢谢！

（参考：https://mongoosejs.com/docs/defaults.html#default-functions-and-this）

编辑2：整个架构已更新

const mongoose = require('mongoose');

const Student = mongoose.model('Student');
const Course = mongoose.model('Course');

let enrolmentSchema = new mongoose.Schema({

        student: {
            type: Number,min: 1000000000,max: 9999999999,required: [true,"missing studentID"],validate:{
                isAsync: true,validator: function(v,cb){
                    Student.find({studentID: v},function(err,docs){
                        cb(docs.length != 0);
                    })
                },message: 'Student does not exist...'
            }

        },course: {type: String,minlength: 3,maxlength: 3,"missing courseID"]},semester:{
            type: Number,validate:
            {
                validator: function(v){
                    return Number.isInteger(v) && v>= 1 && v<= 8;
                },message: "semester must be an integral value from [1 to 8]"
            }
        },finalGrade: {
            type: String,enum: ['E','G','P','F','']
        },numericGrade:{
            type: Number,default: function()
            {
                console.log(escape("This is finalGrade:" +this.finalGrade)); //debugging
                if(this.finalGrade==='E'){
                    return 5;
                }
                else if(this.finalGrade==='G'){
                    return 4;
                }
                else if(this.finalGrade==='P'){
                    return 3;
                }
                else if(this.finalGrade==='F'){
                    return 2;
                }
                else if(this.finalGrade===''){
                    return 1;
                }
                else return 164;
            }
        }

})
mongoose.model('Enrolment',enrolmentSchema);

编辑：根据要求添加了文档创建代码：

function insertEnrolmentRecord(req,res){
    var enrolment = new Enrolment();
    enrolment.student = req.body.student;
    enrolment.course = req.body.course;
    enrolment.semester= req.body.semester;
    enrolment.finalGrade= req.body.finalGrade;
    enrolment.save((err,doc)=>{
        if(err) {
            console.log(err);
            res.redirect('/enrolment/add_failed')
        }
        else res.redirect('/enrolment');

   })
}