When using the onUpdate function in Firebase, how do I retrieve the record that has been updated?

Question

When using the cloud functions on firebase, with the onUpdate trigger, how do I retrieve the record that has been updated (in other words, the record that triggers the function)? I am using JavaScript to interface with the Firebase database.

Answer 1

The first argument passed your onUpdate handler function is a Change object. This object has two properties, before and after, both DataSnapshot objects. These DataSnapshot objects describe the contents of the database before and after the change that triggered the function.

exports.foo = functions.database.ref('/location-of-interest')
.onUpdate((change) => {
    const before = change.before  // DataSnapshot before the change
    const after = change.after  // DataSnapshot after the change
})

Answer 2

per https://firebase.google.com/docs/reference/functions/functions.database.RefBuilder#onUpdate

onUpdate(handler) => function(functions.Change containing non-null functions.database.DataSnapshot, optional non-null functions.EventContext)

So I'm guessing that you just need to pass a callback function into the onUpdate(callback) trigger. Per the documentation, the record of that has been updated would seem to be passed in as the first argument. I would start by logging the arguments object inside the callback function.

The example inside the docs is:

// Listens for new messages added to /messages/:pushId/original and creates an
// uppercase version of the message to /messages/:pushId/uppercase
exports.makeUppercase = functions.database.ref('/messages/{pushId}/original')
    .onCreate((snapshot, context) => {
      // Grab the current value of what was written to the Realtime Database.
      const original = snapshot.val();
      console.log('Uppercasing', context.params.pushId, original);
      const uppercase = original.toUpperCase();
      // You must return a Promise when performing asynchronous tasks inside a Functions such as
      // writing to the Firebase Realtime Database.
      // Setting an "uppercase" sibling in the Realtime Database returns a Promise.
      return snapshot.ref.parent.child('uppercase').set(uppercase);
    });