Node.js MongooseError[MissingSchemaError]：尚未为模型注册架构

我需要一点帮助，我有一个类别模式，如下所示，

let mongoose = require("mongoose");
let schema = mongoose.Schema;
const _ = require("lodash");

let category = new schema(
  {
    // Primary Fields
    categoryName: { type: String, unique: true },
    isArchived: { type: Boolean, default: false },
    isDeleted: { type: Boolean, default: false },
    categoryImage: { type: String },
    categorySlug: { type: String, unique: true },
    createdBy:{
        type: mongoose.Types.ObjectId,
        ref: "Users",
    }
  },
  {
    timestamps: true,
  }
);

let Category = mongoose.model("Categories", category);
module.exports = {
    Category,
};

我正在将此引用用于另一个模式，如下所示

let mongoose = require("mongoose");
let schema = mongoose.Schema;
const { ObjectID } = require("mongodb");

let coupons = new schema({
    city: {type:String},
    categoryId: {
        type: mongoose.Types.ObjectId,
        ref: "categories",
    },
    offerCode: {type:String,unique:true},
    couponName: {type:String,unique:true},
    validFrom: {type:Date},
    validTill: {type:Date},
    visibility: { type:Boolean,default:false},
    issuedFor: {type:String,enum:['BIKE','AUTO','DELIVERY','CHARGING']},
    applicability: {type:String,enum:['1','2','3','4','5','6','7','8','9','10','0']},
    bankType: {type:String},
    isArchived: { type: Boolean, default: false },
    isDeleted: { type: Boolean, default: false },
    createdBy: {
        type: mongoose.Types.ObjectId,
        ref: "Users",
    },
},
{
    timestamps: true,
});
    coupons.index( { "offerCode": 1,"couponName": 1,"validFrom": 1, "validTill":1, "categoryId":1 }, { unique: true } );
    let Coupons = mongoose.model("Coupons", coupons);
    module.exports = {
         Coupons
    };

一切看起来都很好，我可以使用其他模式的引用，但每当我试图从我的代码填充它时，类别模式就会抛出MongooseError.

代码如下所示，

return await Coupons.find(query)
        .populate('categoryId')
        .skip(parseInt(skip)).select(select).limit(parseInt(limit)).sort({ createdAt: -1 })

有没有人能告诉我是什么原因让我迈向MongooseError [MissingSchemaError]: Schema hasn't been registered for model "categories".

请帮帮我.先谢谢你.