如何在 Firebase/Google Cloud Firestore 的集合中获取最新添加的文档？

Question

Using the web/JS platform, I want to retrieve the latest document added to a collection.

1. How do I accomplish this?
2. Do I need to append a timestamp when I save the data?
3. Does the server automatically append a timestamp in the background on doc.add() ?

https://firebase.google.com/docs/firestore/query-data/get-data

 db .collection("cities") // .orderBy('added_at', 'desc') // fails // .orderBy('created_at', 'desc') // fails .limit(1) .get() .then(querySnapshot => { querySnapshot.forEach(doc => { console.log(doc.id, " => ", doc.data()); // console.log('timestamp: ', doc.timestamp()); // throws error (not a function) // console.log('timestamp: ', doc.get('created_at')); // undefined }); });

Answer 1

You could try using the onSnapshot method to listen to change events:

db.collection("cities").where("state", "==", "CA")
    .onSnapshot(function(snapshot) {
        snapshot.docChanges().forEach(function(change) {
            if (change.type === "added") {
                console.log("New city: ", change.doc.data());
                //do what you want here!
                //function for rearranging or sorting etc.
            }
            if (change.type === "modified") {
                console.log("Modified city: ", change.doc.data());
            }
            if (change.type === "removed") {
                console.log("Removed city: ", change.doc.data());
            }
        });
    });

Source: https://firebase.google.com/docs/firestore/query-data/listen