I have cloud function triggered when a new object is added to a collection. It looks like this:

```
exports.emailAdmin = functions.firestore
  .document('user-books/{userId}/books/{ASIN}')
  .onWrite(event => {

```

That `event.data.data()` is an object added to the sub-collection ("books"). The `userId` comes from the Firebase authentication system. I.e. a user signed in, and added an object to the collection "user-books" with his/her "uid".

I tried:

```
firestore
    .collection('users')
    .doc(uid)
    .get()

```

But that, "of course", fails because *I* don't have a collection called "users". How do I get to the "authentication database"?

The purpose is to convert the "uid" to that person's "email".