How to use array type in MongoDB schema (using Mongoose) such that the array consists of _id's of another table

Below is my orderSchema. So I want this orderSchema to save array of Dishes(Items) which basically is another schema (dishSchema). Currently, I am using the commented syntax which basically throwing an error. Any other way to do that?

Thank's in advance.

const mongoose = require("mongoose");

const orderSchema = mongoose.Schema({
    _id: mongoose.Schema.Types.ObjectId,
    cost: { type: String, required: true },
    resId: {
        type: mongoose.Schema.Types.ObjectId,
        required: true,
        ref: "Restaraunt",
    },
    userID: {
        type: mongoose.Schema.Types.ObjectId,
        required: true,
        ref: "User",
    },
    //Items: [{ type: mongoose.Schema.Types.ObjectId, ref: "Dish" }],
    totalItems: {
        type: Number,
        required: true
    }
});
module.exports = mongoose.model("Order", orderSchema);