将文档添加到Firestore集合时触发云功能

我目前正在尝试使用Firestore，SwiftUI和Cloud Functions集成付款处理器。

我目前具有以下功能，该功能允许我将Firestore数据发送到处理器并获得返回值，该返回值将保存到Firestore中。

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp();
const db = admin.firestore();

const Iyzipay = require('iyzipay');

const iyzipay = new Iyzipay({
  apiKey: 'sandbox-afXhZPW0MQlE4dCUUlHcEopnMBgXnAZI',
  secretKey: 'sandbox-wbwpzKIiplZxI3hh5ALI4FJyAcZKL6kq',
    uri: 'https://sandbox-api.iyzipay.com'
});

exports.pay = functions.https.onRequest((req, res) => {
  const docRef = db.collection('requests').doc('zFcWYdeP8wyLprYrc6o1');
  docRef.get().then(doc => {
      if (!doc.exists) {
        console.log('No such document!');
        return res.send('Not Found')
      } 
        console.log(doc.data());
        res.send(doc.data());

       const request = {
        locale: Iyzipay.LOCALE.TR,
        conversationId: '123456789',
        price: doc.data(doc).price,
        paidPrice: doc.data(doc).price,
        currency: Iyzipay.CURRENCY.TRY,
        installment: '1',
        basketId: 'B67832',
        paymentChannel: Iyzipay.PAYMENT_CHANNEL.MOBILE_IOS,
        paymentGroup: Iyzipay.PAYMENT_GROUP.LISTING,
        paymentCard: {
            cardHolderName: doc.data(doc).cardHolderName,
            cardNumber: doc.data(doc).cardNumber,
            expireMonth: doc.data(doc).expireMonth,
            expireYear: doc.data(doc).expireYear,
            cvc: doc.data(doc).cvc
        },
        buyer: {
            id: doc.data(doc).uid,
            name: doc.data(doc).name,
            surname: doc.data(doc).surname,
            gsmNumber: doc.data(doc).gsmNumber,
            email: doc.data(doc).email,
            identityNumber: doc.data(doc).identityNumber,
            registrationAddress: doc.data(doc).registrationAddress,
            city: doc.data(doc).city,
            country: doc.data(doc).country,
            zipCode: doc.data(doc).zipCode
        },
        shippingAddress: {
            contactName: doc.data(doc).name,
            city: doc.data(doc).city,
            country: doc.data(doc).country,
            address: doc.data(doc).registrationAddress,
            zipCode: doc.data(doc).zipCode
        },
        billingAddress: {
          contactName: doc.data(doc).name,
          city: doc.data(doc).city,
          country: doc.data(doc).country,
          address: doc.data(doc).registrationAddress,
          zipCode: doc.data(doc).zipCode
        },
        basketItems: [
            {
                id: doc.data(doc).productid,
                name: doc.data(doc).productname,
                category1: doc.data(doc).category1,
                itemType: Iyzipay.BASKET_ITEM_TYPE.PHYSICAL,
                price: doc.data(doc).price
            },
        ]
       }
       iyzipay.payment.create(request, function (err, result) {
        console.log(err, result);
        
        db.collection('results').doc(doc.data(doc).uid).set(result);
      }) 
      })
      })

我的意图是在将文档添加到"requests"集合中时触发该函数并获取该文档，而无需像这样手动编写其ID：

const docRef = db.collection('requests').doc('zFcWYdeP8wyLprYrc6o1');

我尝试更改functions.https.onRequest((req, res)为functions.firestore.document('/requests/').onCreate((snap, context)，但随后收到：

UnhandledPromiseRejectionWarning: ReferenceError: res is not defined

我该如何解决？谢谢。