猫鼬自定义验证课程的先决条件？

Question

我有以下CourseSchema，

我需要通过以下条件来验证先决条件字段：

-它必须为空，

-或长度为3，并且与现有课程ID匹配

我该如何处理？我需要在这里连接到MongoDB吗？ 预先感谢！

let courseSchema = new mongoose.Schema({
    courseID: {
        type: String, 
        minlength: 3, 
        maxlength: 3, 
        required: [true, "missing courseID"],
        unique: true
    }, 
    courseName: {type: String, required: [true, "missing courseName"]},
    prerequisites: {
        type: String,
        validate:{
            validator: function(v){
                if(v.length === 0){
                    return true;
                }
                else if(v.length === 3){
                    //check if the prerequisite course exist in the DB
                        //if not return false
                        //else return true
                }
            },
            message: "prerequisites must either be empty or (have length= 3 and exist)"
        }
    }
})