Firebase无法使用“快照”访问数据

Question

I'm writing a web application that uses Firebase, and reading information from the database structured like the following:

"user" : {
"-KcNmMmqbSvmVXx2vn4A" : {
  "email" : "firebase@firebase.com",
  "firstname" : "fire",
  "membership_tier" : 1,
  "session_count" : 0,
  "surname" : "base"
}

I'm attempting to read the number of sessions the user has undertaken - however no results are being returned.

What I've tried so far:

 firebase.database().ref('/users/' + uid).once('value').then(function(snapshot) {
   var sessionCount = snapshot.val().session_count;
 });

Currently snapshot.val() is returning null . I'm getting an error stating:

Uncaught (in promise) TypeError: Cannot read property 'session_count' of null

uid has been defined, and therefore no issue should be had with accessing the user's information.

Any pointers would be massively appreciated!

Answer 1

The below works for me:

  //INITIALIZE FIREBASE WEB APP
  firebase.initializeApp(config); 
  var db = firebase.database();
  var auth = firebase.auth();

  db.ref('/whatever/').once('value').then(function(snap) {
    if(snap.val()){
      console.log(snap.val().whateverProperty);
    } else {
      console.log('/whatever/whateverProperty node does not exist!');
    }
  }, function(error) {
    // The Promise was rejected.
    console.log(error);
  });

Answer 2

Looking at the code shared by you, you have defined user in database but trying to access users which could cause the error that you are getting. Make sure you are using correct ref .

Answer 3

Vivek was right that I hadn't used user , and would have also lead to an error, however the main issue was that the reading code resided within a function that had database passed to it, assigned to firebase.database() . Changing the code to this worked fine:

database.ref('/user/' + uid).once('value').then(function(snapshot) {
  var sessionCount = snapshot.val().session_count;
});

Cheers for the pointers!