Firestore query for loop with multiple values

Question

I am attempting to retrieve a number of Firestore documents using data held in a string. The idea is that for each value in the array, i'd use Firestore query to retrieve the document matching that query and push it to another array. I am having a few issues achieving this. So far i've tried:

exports.findMultipleItems = functions.https.onRequest((request, response) => {
    var list = ["item1", "item2", "item3", "item4"];

    var outputList = [];

    for (var i = 0; i < list.length; i++) {
        console.log("Current item: " + list[i]);
        let queryRef = db.collection("items").where('listedItems', 'array-contains', list[i]).get()
            .then(snapshot => {
                if (snapshot.empty) {
                    console.log('No matching documents.');
                }

                snapshot.forEach(doc => {
                    outputList.push(doc.data());
                });
                return;
            })
            .catch(err => {
                console.log('Error getting documents', err);
            });
    }

    response.send(JSON.stringify(outputList));

});

I'm not entirely sure but i think one of the issues is that the for loop is being completed before the queries have a chance to finish.

P.s - this is being ran through Cloud Functions using Admin SDK.

Answer 1

Your queryRef is not actually a reference. It's a promise that resolves after your get/then/catch have finished. You need to use these promises to determine when they're all complete. The array will be populated only after they are all complete, and only then is it safe to send the response using that array.

Collect all the promises into an array, and use Promise.all() to get a new promise that resolves after they're all complete:

exports.findMultipleItems = functions.https.onRequest((request, response) => {
    var list = ["item1", "item2", "item3", "item4"];

    var outputList = [];
    const promises = [];

    for (var i = 0; i < list.length; i++) {
        console.log("Current item: " + list[i]);
        let promise = db.collection("items").where('listedItems', 'array-contains', list[i]).get()
            .then(snapshot => {
                if (snapshot.empty) {
                    console.log('No matching documents.');
                }

                snapshot.forEach(doc => {
                    outputList.push(doc.data());
                });
                return;
            })
            .catch(err => {
                console.log('Error getting documents', err);
            });
        promises.push(promise);
    }

    Promise.all(promises).then(() => {
        response.send(JSON.stringify(outputList));
    }
    .catch(err => {
        response.status(500);
    })

});

You might want to use these tutorials to better understand how to deal with promises in Cloud Functions:

https://firebase.google.com/docs/functions/video-series/