How to Write Data Into Firebase Realtime Database with Cloud Functions when new Data is added to a Specific Child

Question

I have the following realtime database schema:

schema

I'm working on a social app where a user can like another user. When the user clicks on the like button, a new entry will be added to myLikes->userId list

MyLike myLike = new MyLike(userId, System.currentTimeMillis();
         FirebaseDatabase.getInstance().getReference().child("myLikes").child(userId).child(System.currentTimeMillis()).setValue(myLike);

When the new entry is completed, the cloud function gets triggered to write a new entry in whoLikedMe>userId list for the other User who has been liked.

exports.whoLikedMe = functions.database.ref('/myLikes/{userId}')
    .onWrite((change, context) => {
      // Only edit data when it is first created.
      if (change.before.exists()) {
        return null;
      }
      // Exit when the data is deleted.
      if (!change.after.exists()) {
        return null;
      }
      // Grab the current value of what was written to the Realtime Database.
      const data2 = change.after.val();
      const likedDate = data2.date;
      const myUserId = data2.I_Liked_UserId;
      var likedMe = {
            date: likedDate,
            Who_Liked_Me_UserId: myUserId,
        }

      //get the current date in millis  
      var hrTime = process.hrtime();
      const dateMillis = hrTime[0];
      return admin.database().ref('/WholikedMe/'+myUserId+'/'+hrTime[0]).set(likedMe);
    });

The function gets triggered ok but no entries are inserted into the realtime database. What I'm doing wrong?

EDIT:

When I changed:

exports.whoLikedMe = functions.database.ref('/myLikes/{userId}')

with

exports.whoLikedMe = functions.database.ref('/myLikes/{userId}/915170400000')

(which adds the hardcoded timestamp to the path) all works fine. The problem is that the timestamp is constantly changing. any idea how to accomplish this with the timestamp?

Answer 1

You're using wildcard syntax (curly braces) in the following line:

return change.after.ref('/WholikedMe/{myUserId}/{hrTime[0]}').set(likeMe)

That syntax does not apply to a set statement. You need to use:

return change.after.ref('/WholikedMe/' + myUserId + '/' + hrTime[0]).set(likeMe)

... I think, doing this on my phone so can't double check. :-)

Answer 2

I figure d out. In addition to the syntax correction suggested by @GrahamD I had to change

exports.whoLikedMe = functions.database.ref('/myLikes/{userId}')

with

exports.whoLikedMe = functions.database.ref('/myLikes/{userId}/{timestamp')

and now all is working fine.