Firebase functions onCreate() onDelete() path specificity

Question

I'm experiencing a lack of understanding of Firebase functions events. To make my function work as intended, I am adding more specificity to the reference path then I believe is required.

For example, I want to increment or decrement a count each time a child is added to or deleted from this path:

/invites

I used these as my functions to accomplish this:

functions.database.ref('/invites').onCreate(event => {
  return admin.database().ref('/counter').transaction((current) => {
    return (current || 0) + 1;
  })
});

functions.database.ref('/invites').onDelete(event => {
  return admin.database().ref('/counter').transaction((current) => {
    if (current >=1) return (current - 1);
    else return;
  })
});

This is what I based my path on:

Path specifications match all writes that touch a path, including writes that happen anywhere below it. If you set the path for your function as /foo/bar, it matches events at both of these locations:

/foo/bar

/foo/bar/baz/really/deep/path

In my case /invites would be the equivalent of /bar in that example.

However, the above functions only fire on the first child being added to /invites as well as the last child being deleted. Outside of those two conditions, anything else will not trigger the counter functions. The only way I have been able to make this work is by using this as my path:

invites/{idOfInvite}

While I understand this would work, I don't know that this is exactly what I need. Later I might decide to add new data under the path of a certain {idOfInvite} , which would cause my counter function to fire (I think?).

This SO answer indicates that my functions' behavior is expected, but it is not what I expect based on my understanding of the docs. Perhaps I'm interpreting the docs incorrectly?

As you can tell, my understanding of how these add/delete events trigger is somewhat off.

Answer 1

onCreate will only be called only when the given path is newly created in the database. If the path already exists (that is, it already has a child), then adding another child is not a "create" event at that path. It's an update event. So, if your function was using onUpdate , it will get triggered anytime anything under that path is written to.

Your use of onCreate/onDelete at /invites/{id} is correct for you use case, because it's effectively asking to be called when any individual child of /invites is newly created or deleted.