如何从嵌套的父firebase中获取user_id

{
  "users": {
    "user_id_1": {
      "name": "John",
      "email": "john@example.com",
      "nestedFirebase": {
        "user_id": "user_id_1"
      }
    },
    "user_id_2": {
      "name": "Jane",
      "email": "jane@example.com",
      "nestedFirebase": {
        "user_id": "user_id_2"
      }
    }
  }
}

const user = firebase.auth().currentUser;
const userId = user.uid;

const firebaseRef = firebase.database().ref(`users/${userId}/nestedFirebase`);
firebaseRef.once('value', (snapshot) => {
  const nestedFirebaseData = snapshot.val();
  const nestedUserId = nestedFirebaseData.user_id;
  console.log(nestedUserId);
});