Cache first implementation for get in firestore

Question

I want firestore to fetch data first from cache every time. As per Firestore documentation passing "cache" or "server" options must enable the same. Example below

db.collection("cities").where("capital", "==", true)
    .get("cache")
    .then(function(querySnapshot) {
        querySnapshot.forEach(function(doc) {
            // doc.data() is never undefined for query doc snapshots
            console.log(doc.id, " => ", doc.data());
        });
    })
    .catch(function(error) {
        console.log("Error getting documents: ", error);
    });

However whether i set "cache" or "server", query seems to try network first followed by cache.

Answer 1

Since a get() only gets your a value once, it will always check for the latest value for the data from the server. If this is the first Firestore operation in your app, this may require that it establishes the network connection to the database, which may take some time.

If you quickly want to get the data from the cache, and then later get the modifications from the server, use onSnapshot:

db.collection("cities").where("capital", "==", true)
    .onSnapshot(function(querySnapshot) {
        querySnapshot.forEach(function(doc) {
            // doc.data() is never undefined for query doc snapshots
            console.log(doc.id, " => ", doc.data());
        });
    })
    .catch(function(error) {
        console.log("Error getting documents: ", error);
    });

Also see the Firestore documentation on getting realtime updates

To tell get() to return data from the cache, do:

db.collection("cities").where("capital", "==", true)
    .get({ source: 'cache' })
    .then(function(querySnapshot) {
        ...

But note that this means you will never get the updated value from the server.