Firebase Web retrieve data

Question

I have the following db structure in firebase

I'm trying to grab data that belongs to a specific user id (uid). The documentation has the following example:

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

But how can I retrieve data from my example without knowing the unique key for each object?

Update:

I tried a new approach by adding the user id as the main key and each child object has it's own unique id.

Now the challenge is how to get the value of "title".

Solution

firebase.database().ref('/tasks/').orderByChild('uid').equalTo(userUID)

Well that is pretty straightforward. Then you can use it like this:

return firebase.database().ref('/tasks/').orderByChild('uid').equalTo(userUID).once('value').then(function(snapshot) {
  var username = snapshot.val().username;
  // ...
});

Of course you need to set userUID.

It is query with some filtering. More on Retrieve Data - Firebase doc

Edit: Solution for new challenge is:

var ref = firebase.database().ref('/tasks/' + userUID);
//I am doing a child based listener, but you can use .once('value')...
ref.on('child_added', function(data) {
   //data.key will be like -KPmraap79lz41FpWqLI
   addNewTaskView(data.key, data.val().title);
});

ref.on('child_changed', function(data) {
   updateTaskView(data.key, data.val().title);
});

ref.on('child_removed', function(data) {
   removeTaskView(data.key, data.val().title);
});

Note that this is just an example.