How can I make a Promise in the correct scope so that it has the correct values to query with and also returns?

Question

I'm trying to send a notification with Firebase Cloud Functions when someone likes a photo. I've coped the Firebase example showing how to send one when someone follows you and tried to modify it.

The problem is that I need to do another additional query in the function to get the key of the person who liked a photo before I can get their token from their User node. The problem is that the getDeviceTokensPromise somehow is not fulfilled and tokensSnapshot.hasChildren can't be read because it is undefined. How can I fix it?

    exports.sendPhotoLikeNotification = functions.database.ref(`/photo_like_clusters/{photoID}/{likedUserID}`)
        .onWrite((change, context) => {
          const photoID = context.params.photoID;
          const likedUserID = context.params.likedUserID;
          // If un-follow we exit the function
          if (!change.after.val()) {
            return console.log('User ', likedUserID, 'un-liked photo', photoID);
          }

          var tripName;
          var username = change.after.val()

          // The snapshot to the user's tokens.
          let tokensSnapshot;

          // The array containing all the user's tokens.
          let tokens;

          const photoInfoPromise = admin.database().ref(`/photos/${photoID}`).once('value')
            .then(dataSnapshot => {
              tripName = dataSnapshot.val().tripName;
              key = dataSnapshot.val().ownerKey;
              console.log("the key is", key)
              return getDeviceTokensPromise = admin.database()
              .ref(`/users/${key}/notificationTokens`).once('value');
    ///// I need to make sure that this returns the promise used in Promise.all()/////
            });


          return Promise.all([getDeviceTokensPromise, photoInfoPromise]).then(results => {
            console.log(results)
            tokensSnapshot = results[0];

            // Check if there are any device tokens.

//////This is where I get the error that tokensSnapshot is undefined///////
            if (!tokensSnapshot.hasChildren()) {
              return console.log('There are no notification tokens to send to.');
            }

            // Notification details.

            const payload = {
              notification: {
                title: 'Someone liked a photo!',
                body: `${username} liked one of your photos in ${tripName}.`,
              }
            };

        // Listing all tokens as an array.
        tokens = Object.keys(tokensSnapshot.val());
        // Send notifications to all tokens.
        return admin.messaging().sendToDevice(tokens, payload);
      });

Answer 1

Ok so based on your comment, here's the updated answer.

You need to chain your promises and you don't need to use Promise.all() - that function is used for a collection of asynchronous tasks that all need to complete (in parallel) before you take your next action.

Your solution should look like this:

admin.database().ref(`/photos/${photoID}`).once('value')
    .then(dataSnapshot => {
        // Extract your data here
        const key = dataSnapshot.val().ownerKey;

        // Do your other stuff here
        // Now perform your next query
        return admin.database().ref(`/users/${key}/notificationTokens`).once('value');
    })

    .then(tokenSnapshot => {
        // Here you can use token Snapshot
    })

    .catch(error => {
        console.log(error);
    });

Because your second network request depends on the completion of the first, chain your promises by appending a second .then() after your first and don't terminate it with a semi-colon ;. If the new promise created within first .then() scope resolves it will call the second .then() function.

If you need to access variables from the first .then() scope in the second, then you must declare them in the general function, and assign them in the closure scopes.

You can check out more info on promise chaining here.