const UserSchema = new Schema({
    userId:Schema.Types.ObjectId,
    username: String,
    mobile: String,
    password: String,
    //
    remarks: String,
    createAt: {type: Date, default: Date.now},
    updateAt: {type: Date, default: Date.now},
    //
    userType: String
})
 this is the user schema, I defined. How do I set the userId value to the default objectId, when I want to default inch data? 
 when I query, I need to query the data according to the user id or objectId, and how to get the objectId? 
the following is the code for me to store the data:
let password = md5(ctx.request.body.password)
            let UserModel = new User({
                username: ctx.request.body.username,
                mobile: ctx.request.body.mobile,
                password: password,
                remarks: ctx.request.body.remarks
            })
            UserModel.save(function (err) {
                if (err) console.log(err)
            })