Schema hasn't been registered for model "Item". code example

Example 1: Schema hasn't been registered for model "products"

const mongoose = require("mongoose");
const Schema = mongoose.Schema;
//product model is not imported in orders model
const orderSchema = new Schema(
  {
    // _id : mongoose.Schema.Types.ObjectId,
    product: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "products",//not Product
      required: [true, "product id is required"],
    },
    quantity: { type: Number, default: 1, min: 1 },
  },
  { versionKey: false, timestamps: true }
);

module.exports = mongoose.model("orders", orderSchema);

// in Orders route, no import of product is required
const orders = require("../models/orders");
const products = require("../models/products");

orders.find()
    .select("quantity product _id")
    .populate({ path: 'product', model: 'products' })
	.exce().then().catch()

Example 2: Schema hasn't been registered for model "products"

const mongoose = require("mongoose");
const Schema = mongoose.Schema;
const products = require("../models/products");
//products model is imported
const orderSchema = new Schema(
  {
    // _id : mongoose.Schema.Types.ObjectId,
    product: {
      type: mongoose.Schema.Types.ObjectId,
      ref: products, //not "products"
      required: [true, "product id is required"],
    },
    quantity: { type: Number, default: 1, min: 1 },
  },
  { versionKey: false, timestamps: true }
);

module.exports = mongoose.model("orders", orderSchema);

// in Orders route, no import of product is required
const orders = require("../models/orders");
orders.find()
    .select("quantity product _id")
    .populate("product", "name price")
	.exce().then().catch()