如何检查 Firestore 查询是否为空或我尝试获取的文档不存在？

Question

I am trying to fetch a document in Cloud Firestore based on a field ID, but that ID cannot be found, how to check that because it is throwing an error?

    getMenu = () => {
    try {
        this.setState({
            loading_menu: true
        })

        var ref = firebase.firestore().collection('restaurants_menu')
            .where("rest_id", "==", this.getSelected.rest_id)
            .limit(this.state.limit_menu)

        ref.onSnapshot((querySnapshot => {
            var menu = querySnapshot.docs.map(doc => { return { ...doc.data(), id: doc.id } });
            var lastVisibleMenu = menu[menu.length - 1].rest_id;
            this.setState({
                menu: menu,
                lastVisibleMenu: lastVisibleMenu,
                loading_menu: false,
            });
        }));
    }
    catch (error) {
        console.log("SOMETHING HAPPENED, WE COULD NOT FETCH restaurants_menu:  ", error);
    }
}

Answer 1

If your snapshot handler is getting invoked, then there was no error. Just check the length of the querySnapshot.docs array to find out how many documents were matched by the query. It's not an error to match 0 documents - you should expect that could be the case.

If you want to handle possible errors, you should be passing more than a single snapshot handler. The documentation for onSnapshot() suggests that you should pass a single observer object with various properties for each type of handler function.

ref.onSnapshot({
    next: snapshot => {
        // your snapshot handler code
    },
    error: error => {
        // your error handling code
    }
})