Using Firebase Cloud Functions to Manage a Compound Key Index

index.js

/**
 * Function: indexArtist
 *
 * Triggered by writes to: /artists/:id
 * Responds by writing to: /indexes/artist_by_genre (once for each Genre the Artist has chosen)
 *
 * If there was a previous value, all associated index entries are removed first.
 * So, if the artist display name has changed or the genre list has changed,
 * the index will be properly synchronized.
 *
 * See the explanatory article here:
 * http://cliffordhall.com/2017/04/using-firebase-cloud-functions-manage-compound-key-index/
 *
 * @type {CloudFunction<any>|CloudFunction<DeltaSnapshot>}
 */
exports.indexArtist = functions.database.ref('/artists/{uid}')
    .onWrite(event => {

        // All the write promises this function will create
        let writes = [];
        let artistByGenreIndex = admin.database().ref('/indexes/artist_by_genre');

        console.log('Triggered by artist', event.params.uid, event.data.val().user.name.display);

        // Remove any previous entries since genre or display name may have changed
        var previous = event.data.previous.exists() ? event.data.previous.val() : null;
        let deletes = [], key, val;
        if (previous && previous.genres){
            console.log('Removing old index entries...');
            previous.genres.forEach( genre => {
                key = genre.name + previous.user.name.display;
                deletes.push( artistByGenreIndex.child(key).set(null) );
            });
            console.log(deletes.length+' entries removed.');
        }

        // If any previous entries were deleted, wait until they finish
        Promise.all(deletes).then(addKeys).catch(addKeys);

        // You must return a Promise when performing async tasks in a Firebase Cloud Function
        return Promise.all(writes);

        // Add the keys for all Genres this Artist has chosen
        function addKeys() {
            const current = event.data.exists() ? event.data.val() : null;
            if (current && current.genres) {
                console.log('Indexing artist: ', event.params.uid, current.user.name.display);
                current.genres.forEach( genre => {
                    key = genre.name + current.user.name.display;
                    val = {key: key, artist: {user: current.user}, genre:genre};
                    writes.push( artistByGenreIndex.child(key).set(val) );
                });
            }
        }
    });