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,"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)"
        }
    }
})