如何在猫鼬中建立一对多关系

Question

我正在学习mongoDB和mongoose，我正在尝试在客户和订单之间建立一对多关系 但出现一个无法创建之前访问orderSchema的错误。请帮助我解决该错误。这是我的代码：

    const customerSchema = new mongoose.Schema({
  _id: {
    type: Number,
    required: true
  },
  
  name: {
    type: String,
    maxlength: 50,
  },

  email: {
    type: String,
    maxlength: 50,
  },

  orders: orderSchema,
});

const productSchema = new mongoose.Schema({
  _id: {
    type: Number,
    required: true,
  },

  name: {
    type: String,
    maxlength: 50,
  },

  price: {
    type: Number,
  }
});

const orderSchema = new mongoose.Schema({
  order_id: {
    type: Number,
    required: true,
  },

  product: productSchema,
  customer: customerSchema
});