错误：.find(…) 不是函数 nodejs mongoose

我定义了一个控制器，其代码如下：

const  paymentModel = require("../../models/University");
class PaymentController {
async getPaymentList(req , res){
   const list = await paymentModel.find() 
   .select("amount  description  payDate payId" ).limit(20);
   res.send(list);
}

路由器如下：

const router = require ("express").Router();
const controller = require("../http/controller/PaymentController");
router.get('/' ,controller.getPaymentList);
module.exports= router;

模型 ：

 const mongoose = require ("mongoose");
 const schemaPay = new mongoose.Schema({
 payId  :{type:Number, required: true},
 amount :{type :String , required: true},
 description :{type :String , required:true},
 courseId:[schemaCourse] ,
 studentId:[schemaStudent],
 payDate :String ,
 });
 const paymentModel= mongoose.model('StudentPayments' , schemaPay); 
 module.exports={paymentModel };

当我用邮递员测试 api 时，我收到错误：paymentModel.find 不是一个函数，有人可以帮我解决问题吗？