如何用DocumentSnapshot代替QuerySnapshot？

const Firestore = require('@google-cloud/firestore');
const firestore = new Firestore();

const docRef = firestore.collection('collectionName').doc('docId');

docRef.get()
  .then((snapshot) => {
    if (snapshot.exists) {
      const documentData = snapshot.data();
      // 处理文档数据
    } else {
      // 文档不存在
    }
  })
  .catch((error) => {
    console.error('Error getting document:', error);
  });

const Firestore = require('@google-cloud/firestore');
const firestore = new Firestore();

const collectionRef = firestore.collection('collectionName');

collectionRef.where('field', '==', 'value')
  .get()
  .then((snapshot) => {
    snapshot.forEach((doc) => {
      const documentData = doc.data();
      // 处理文档数据
    });
  })
  .catch((error) => {
    console.error('Error getting documents:', error);
  });